What I take you to be asking about, Ben,
is one of the unsourced "Agrapha" of Jesus that shows up in early Church writers, in this case Origen. You are noticing that Origen said "it is written", and Origen doesn't say where, and that it also shows up in the "Gospel of Thomas". You quoted Origen as saying:
Many have taken in hand to write, but four gospels only are approved, from which the dogmas about the person of our Lord and savior are to be derived. I know a certain gospel which is called according to Thomas, and one according to Matthias, and many others we read, lest we should be seen as ignorant on account of those who suppose they know something if they have knowledge of those.
In that quote, Origen does not specify whether all the sayings in the Gospel of Thomas are mistaken, false, or heretical. He says that "we read" it, and Origen never says where "it is written" when he quotes a saying that *happens* to be in the Gospel of Thomas. Certainly, since Origen read the Gospel of Thomas, he would have read the saying in it that you quoted about being near fire. Trying to put the matter simply:
1. Origen read the Gospel of Thomas.
2. Origen read the quote in it about being near fire.
3. Origen said "It is written..." and then quoted that saying about being near fire.
The logical conclusion is that Origen was repeating a saying that he had read in the Gospel of Thomas. And one reasonable explanation is that he did not cite where "it is written" because that citation would undermine the authority of the saying that he was quoting. My guess is that the phrase "it is written" does not always mean that an early Christian writer believed that the source was a Biblical canon or otherwise ecclesiastically authoritative, but I would have to double-check on that.
Of course, this is not the only explanation. Origen could have read the same quote somewhere else like in another apocryphal Gospel. But AFAIK, this is the first place we have in our remaining texts, so the most likely answer would be that Origen found the saying there.
BTW, here is the Early Writings Commentary page on that Saying:
http://www.earlychristianwritings.com/t ... mas82.html