That’s a tricky one for me because I see a lot of utility in asking for sources:
a) Curiosity. Maybe I really want to know how this person came to believe what they did even if I’m pretty sure they’re wrong.
b) Challenge. A version of the Socratic Method, helping them learn through questioning their assumptions.
c) Learning. A genuine quest for knowledge, that perhaps might yield more unique or varied results than Google.
d) Depth. Possibly helping others in a public thread (or the author) realize that it might be an epistemically illegible argument requiring greater scrutiny, discussion or evidence.
I get that asking for sources can also be lazy or ill intentioned, but for the most part I don’t mind it.