When One's Hat Falls off in Salaah

Q: Salaah-415: What is one to do if one's hat falls off while one is performing salaah? Should it be picked up and worn?

Answer: It is not permissible to pick up one's hat when it falls off during Qiyaam or Ruku since this will constitute an excessive act (Amal Katheer) which will nullify the salaah. If it falls in front of the spot where one makes Sajdah, one may replace it with a slight action, such as using one hand only. Not only is this allowed, but encouraged. The salaah will not be affected by it And Allaah knows best what is most correct.

Fatawa Rahimiyyah vol.2