It is narrated from Sayyiduna Wa'il bin Hujr (may Allah be pleased with him) that he said: "I prayed with the Messenger of Allah (peace and blessings be upon him), and I saw him (peace and blessings be upon him) placing the right hand on the other (the left hand)."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18271