Musannaf Ibn Abi Shayba 3:1559

Waki’ narrated to us, from Isma’il bin Abi Khalid, from al-A’mash, from Mujahid, from Muwarriq al-‘Ijli, from Abu al-Darda’ who said: "From the character of the prophets is placing the right hand upon the left in prayer."