Is cracking one's knuckles Haram? Is it mentioned in Hadith?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

If you mean by your question cracking finger knuckles during the prayer, then it is disliked as stated by the jurists and some of them are even of the view that this is forbidden.

If it is outside the prayer, it is also disliked by some scholars except if there is a need for doing so.

Therefore, it is better to totally avoid it if there is no need for doing so.

Allaah Knows best.