The answer is 'The Holy Traditions' of The One Holy Catholic and Apostolic Church. What makes them Holy is that Christ taught them to the Apostles.
They are the equal to scripture in Christian Authority. In fact, without them scripture would not have be regarded with any authority. The history of how these books were accepted into Christianity by the bishops over the first 300-400 years shows this clearly. The New Testament was gathered and assembled by the Church Fathers, for the purpose that each and every Christian community would have a copy of the same books and that they would all use them in unison on service days... a service in one part of the world would have the same content as the services in another.
I would also challenge the idea that scripture is infallible. That in itself is an extra-biblical idea. Clearly contradictions can be shown. (day of preparation, Matt, Mark, Luke vs. John)
Nor does scripture ever state that it is to be used alone. In fact that is an absurd idea. One would have to strip all references to Christian liturgical worship services from scripture in order to do so.
Can we show from scripture how the Psalms are to be used in Christian service? Christians continued to use them is liturgical Christian worship as they had when they were Jewish. As witness to that, they are still in the middle of our bibles. When did they get dropped from use and why?
God be gracious to me a sinner.