Well we definitely have material from both the Apostolic and Early Church Fathers.   The earliest reference is in the Shepherd of Hermas, which is dated to the late first century., and which is by no means heretical - St. Athanasius of Alexandria officially approved of the use of this book for edification and moral instruction of catechumens (converts awaiting Baptism), alongside the 27 book New Testament canon that he introduced in his 39th Paschal Encyclical in the mid 4th century, which we still use today.  Indeed the 27 book New Testament canon and the Nicene Creed are both largely attributable to St. Athanasius, whose importance to the preservation of the Apostolic faith cannot be overstated.  He also wrote two very important books, On The Incarnation, which defends the doctrine of the Incarnation of God in the person of Jesus Christ against Arianism, and which is spectacularly well written and brilliant (for instance, it anticipated a limitation of the Big Bang theory that physicists still have not conclusively solved, that being the non-uniform distribution of matter and energy in the universe), and The Life of Anthony, which is a biography of the Coptic (Egyptian) Christian hermit St. Anthony the Great.
Most of the Patristic references however start in the 3rd century, presumably because of the tremendous expansion of the Church in the third century and the dramatic increase in the number of martyrs and confessors, but there are some references in various works from the second century, for example, the apocryphal Acts of John, which falls into a category of early church literature that is somewhere in between the heretical Gnostic gospels and pious fan-fiction, and in church inscriptions, and other sources, and also very significantly we do not see the intercession of the saints mentioned as a heretical practice in the exhaustive second century catalogue of heresies, Against Heresies, by St. Irenaeus of Lyons.   We also see the non-veneration of the Theotokos by a sect called the Antidicomarians, and the inappropriate worship of the Theotokos by a sect called the Collyridians, condemned as heresies in the even more exhaustive fourth century encyclopedia of heresies, the Panarion (a Greek word that literally translates as “medicine chest”, but which for practical purposes means “First Aid Kit”), by St. Epiphanius the bishop of Salamis, and we see this reiterated in the cataolog of heresies in the Fount of Knowledge of St. John of Damascus, written in the eighth century AD.
Most importantly, however, we have the earliest surviving manuscripts of the oldest Eucharistic liturgies that have survived in continuous use, the traditional Holy Communion service used in Alexandria and the Egyptian church, known to the Greek Orthodox as the Divine Liturgy of St. Mark and to the Coptic Orthodox as the Divine Liturgy of St. Cyril (because in the fifth century, St. Cyril the Great translated the liturgy into the Coptic language in order to evangelize the remaining adherents of Egyptian paganism, who for the most part, with the exception of a few wealthy neo-Platonists such as Hypatia, were rural peasants further up the Nile from Alexandria, or in the desert, who did not speak Greek or benefit from much in terms of education).  We have an actual second century fragment of the Greek version of this liturgy, the Strasbourg Papyrus.  And this liturgy does feature the customary intercessory petitions to the saints one finds in Orthodox liturgies.  The same is also true of the Divine Liturgy of Addai and Mari, which has been authoritatively dated by scholars of the liturgy to no later than the second century, with some arguing for a first century dating, based on the manner in which the liturgy is structured after a Jewish table blessing.  This liturgy is the traditional liturgy of the Church of the East, and is the most widely used of the four extant liturgies of the East Syriac form.
Additionally intercessions of the saints are included in the ancient Divine Liturgies of St. Basil and St. James, and the sample liturgy in the Apostolic Constitutions, not to be confused with the oft-misunderstood liturgy in the Apostolic Tradition of Hippolytus, which appears to be extremely minimalistic, until one realizes that it contains only the parts of the liturgy said by the bishop, and is the main liturgy used by the Ethiopian Orthodox, but in a more complete form, where it is known as the Liturgy of the Apostles.  It is related to the traditional liturgy of Antioch, because a group of seven priests from the region of Antioch travelled to Ethiopia after that country converted to Christianity shortly after the conversion of the Kingdom of Armenia and the legalization of Christianity in the Roman Empire under St. Constantine (who had a Christian mother, St. Helen).  This liturgy, as well as its Antiochene relative, also feature intercessions of the saints, and we also see this in the fourth century derivative of this liturgy, the Divine Liturgy of St. John Chrysostom, which is now the main liturgy in the Eastern Orthodox Church.  The Divine Liturgy of St. Basil was probably adapted by the great fourth century theologian for use in Constantinople and in Egypt (for there are two separate versions of it, a Byzantine version and an Egyptian version, the latter now being the main liturgy in the Coptic Orthodox Church), and the source for it is most likely the Divine Liturgy of St. James, which is the traditional liturgy associated with the Holy Land, which is also given a very early dating, however, unlike the Divine Liturgy of St. Mark, we do not have any second century papyri to confirm the date, and it is also now considered possible the Divine Liturgy of St. James was actually derived from the Divine Liturgy of St. Basil.  Either way, these are very early liturgies.
We also have enough evidence to date the traditional Roman Mass, which probably has more intercessory petitions to the saints than  any other ancient liturgy, to at least the fourth century, with some indications it was used at least since the translation of the liturgy and the Bible from Greek to Latin under Pope Victor in the second century.  And given the large amount of Greek phraseology in the Roman Mass that was not translated, it seems quite possible that the Roman Mass was directly based on an earlier Greek liturgy that was slowly phased out.   However, the most common liturgical form in antiquity was that used in Antioch, for this was the form used in Byzantium, and as far as we know, most of Greece, and this was also the form exported to Armenia, Georgia and Ethiopia, and also the Divine Liturgy of St. James follows the Antiochene pattern, so it makes sense that St. Hippolytus, in his work The Apostolic Tradition, would include an Antiochene liturgy as the exmplary form.  The Roman church however was historically regarded as the most conservative of the ancient churches, with the only major change from the time of St. Clement of Rome in the first century until the controversial Pope Leo in the Fifth Century being the translation of the Bible and the Liturgy into Latin by Pope Victor, since only the upper class spoke Greek.
Now, it is in the third century where we really start to see a lot of references to the practice of intercessory prayer; indeed it would not be an overstatement to say that every major third century theologian endorsed the practice.
I myself also count as Early Church Fathers the fourth century Fathers, both Ante-Nicene and post-Nicene, and also the Fifth Century  Fathers, indeed, I personally regard the period up to the Council of Chalcedon and the persecution of the Oriental Orthodox by Justinian as representing the Early Church, basically, the first 500 years, and then the next 500 years I regard as the adolescent blossoming of the Church, still in the Patristic era, to be sure, indeed, the Orthodox do not regard the Patristic era as having ended and still conduct theology in the Patristic manner, in contrast to the Roman Catholics, who regard St. John of Damascus as the last Patristic theologian, since after that point Roman Catholic theology changed course dramatically and entered into what is now called Scholastic theology, which is epitomized by the Summa Theologica of Thomas Aquinas, which is an interesting work, but in my opinion it was not really needed, since everything it covered had been previously addressed in a more succinct form by St. John of Damascus and in earlier works of dogmatic theology, including works by the aforementioned heresiologists (theologians specializing in the study of heresy) St. Irenaeus of Lyons and St. Epiphanius of Salamis.   I am of the opinion that all great works of dogmatic or systemic theology start with the study of heresy, and include material refuting heresy; this is even true of works that I disagree with, such as Calvin’s Institutes.
That said, I gather you are more interested in Ante-Nicene material, and a substantial amount has been curated on this Eastern Orthodox article, which is academically rigorous.
	
	
		
			
				
			
			
				
				It is commonly claimed that the practice of praying to departed saints and to angels is a late development in Christianity, probably post-dating the Council of Nicea. In this post, I will try to ar…
				
					
						
							
						
					
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If you want more quotes, there is a Catholic.com article I can link you to, but I am searching for a better supplemental article as frankly the academic quality of the catholic.com article is not what it should be, to the point where I am surprised it managed to get an imprimatur.  Additionally there are several quotes I recall which are missing from that article, and I want to find an article that includes more of the remarks on this subject which I studied at length in seminary as I became progressively more fascinated with Patristics.