What is the ruling on cracking one's knuckles? Thank you.

Fidgeting during prayer is not permissible, and this includes cracking one's knuckles. Scholars have explicitly stated its detestability (karāhah), such as Ibn Qudamah in Al-Mughni, the author of Tabyin al-Haqa'iq, and Khalil al-Maliki. Imam Malik considered it absolutely detestable, both inside and outside of prayer, and whether in the mosque or elsewhere.