I'm quite sure that this question is a duplicate, but of what?

Maybe if you look through the linked questions, you might find something suitable.

To find that post, I have tried to search for "[decimal-expansion] [elementary-number-theory] [divisibility] 9 is:q" math.stackexchange.com/…

Maybe omitting is:q would make sense too. Maybe also simply choosing those three tags and then try to sort by votes or check the frequent tag would work.

It is kind of easy to suggest alternative ways to find some specific posts after it was already found in some other way. :-)

@MartinSleziak I think that the first one will do just fine.