Maliks Muwatta Book 9, Hadith Number 50.

Section : Placing One Hand on the Other in the Prayer.

Yahya related to me from Malik from Abu Hazim ibn Dinar that Sahl ibn Sad said, “People used to be ordered to place their right hands on their left forearms in the prayer.”

Abu Hazim added, “I know for sure that Sahl traces that back to the Prophet, may Allah bless him and grant him peace.”