Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.131.

This is a preferred act of the prayer. There are twenty hadith from eighteen companions and their followers on this point. Said Sahl ibn Sa’d, “The people were ordered to place their right hand on their left forearm during prayers.” Commenting on this, Abu Hazm says, “I do not know if he ascribed this to the Prophet.” This hadith is related by al-Bukhari, Ahmad and Malik in his al-Muwatta. Al-Hafez maintains, “Its ruling is considered to be from the Prophet, upon whom be peace, as it is implied that the one who ordered them to do so was the Prophet.” He also related that the Prophet said, “All prophets have been ordered to hasten the breaking of the fast and to delay the (pre-fast dawn) meal, and to place our right hands on our left during prayer.”

There is also a hadith from Jabir which says, “The Prophet, upon whom be peace, passed by a man praying with his left hand over his right, and (the Prophet) pulled them away and put his right over his left.” This is related by Ahmad and others. Evaluating its chain, an-Nawawi says, “Its chain is sahih. Ibn ‘Abdul-Barr holds, “Nothing has reached me different from that. It is the opinion of most companions and their followers.” Malik mentioned it in his al-Muwatta and states, “Malik never stopped doing it until he met Allah.”