The Didache mentions fasting before baptism
And before the baptism let the baptiser and him who is to be baptised fast, and any others who are able. And thou shalt bid him who is to be baptised to fast one or two days before.
This practice is attested as early as the first half of the third century, by
Tertullian and particularly
Hippolytus.
I also found this:
Tertullian addresses the topic tangentially while addressing the dangers of women marrying non-Christians. Their husbands will notice their Christian practices, such as fasting before taking communion, and may put pressure on them to stop:
Will not your husband know what it is which you secretly taste before (taking) any food? and if he knows it to be bread, does he not believe it to be
that (bread) which it is
said to be? (
To His Wife, II.5)
Hippolytus is more direct. In the
Apostolic Tradition, he writes:
The faithful shall be careful to partake of the eucharist before eating anything else. (
36)
At the Synod of Hippo (AD 393), the practice is codified in
Canon 29 (Mansi, III, 913), and again a few years later at the Synod of Carthage (
Canon 28 (Mansi, III, 884)). And around this time,
Augustine indicates that the practice is universal:
It pleased the Holy Spirit to appoint, for the honour of so great a sacrament, that the body of the Lord should take the precedence of all other food entering the mouth of a Christian; and it is for this reason that
the custom referred to is universally observed. (
Letter 54, VI.8)