Musnad Ahmad 5:312

'Abdullah narrated to us, Muhammad ibn Sulayman al-Asadi Luwayn narrated to us, Yahya ibn Abi Za'idah narrated to us, 'Abd al-Rahman ibn Ishaq narrated to us, from Ziyad ibn Zayd al-Sawa'i, from Abu Juhayfah, from 'Ali, who said: "Indeed, it is from the Sunnah in prayer to place the palms upon the palms below the navel."