by Imam Malik, translation by A'isha `Abdarahman at-Tarjumana and Ya`qub Johnson , Volume: Shortening the Prayer
Chapter: Placing One Hand on the Other in the Prayer
Yahya related to me from Malik that Abd al-Karim ibn Abi'l-Mukhariq al-Basri said, "Among things the Prophet, may Allah bless him and grant him peace, said and did are: 'As long as you do not feel ashamed, do whatever you wish', the placing of one hand on the other in prayer (one places the right hand on the left), being quick to break the fast, and delaying the meal before dawn."
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."