The crest has been seen on the inside cover of various editions of Harry Potter and the Philosopher's stone.
For added "canon" weight, there's a signed copy available here for just £4950
JKR also hand-drew her own version of the crest in a volume that she later auctioned for charity. It fetched £150,000 if you happen to be looking for an investment
Interestingly, the very first instance of the motto seems to be on the Official Harry Potter Fan Club certificates. I've been unable to find a scanned copy of one, but there's a mention here in a Telegraph interview from 1997.
and in the interview below, she discusses why she chose it.
SF: And names like that. Even the school crest is something which is rather fun for those of us who have done a bit of Latin. For
instance: “Draco dormiens nunquam titillandus” – it’s sort of like
“Let sleeping dogs lie”, but it is “Don’t tickle a sleeping dragon”.
JKR: “Don’t tickle a sleeping dragon”, exactly.
SF: Which is fine advice.
JKR: I wanted good practical advice. All the schools I’ve ever been to or taught in have mottos: “Persevere”, “Onwards and Upwards”.
I wanted good solid practical advice for Hogwarts