How to cite something when you cannot remember where you've read it?
Since you're already here, an obvious step would be to post a question on a relevant Stack Exchange site. (Other websites are available.) Such questions are routinely asked and answered on MathOverflow, for example.

Similarly, asking colleagues can also work. They don't necessarily need to be people you know personally either. There's often someone in a field with a reputation for a "near-encyclopedic knowledge of the literature" you could send an email to.

However, if nothing works, honesty is the best approach. Just present it as a known result of unknown origin, or something to that effect. Here's an example where that kind of formulation is used for a problem, not a result, but the same idea applies.