Did any ancient source believe in more than four gospels?

I'm listening to a lecture series by Dale Martin (Professor of Religious Studies, Yale), and he makes the passing claim that during development of the New Testament canon there were some who thought that four gospels should be considered authoritative, but some thought five, and some six. I can't seem to find any ancient source to back this up. Certainly more than four gospels existed, but are there ancient sources that equates any of them as on par with the four canonical gospels? A list that includes the Gospel of Thomas along side Luke? Or a document that freely quotes from Mark and the Gospel of Peter? I guess 2 Clement is a possibility, but given the author's usage of the Gospel of Thomas and possibly the Gospel of Peter, perhaps they are better examples.