Musnad Abu Ya'la al-Mawsili 109:11

2010. Ishaq narrated to us, Abd al-Razzaq narrated to us, from Mansur, from Salim ibn Abi al-Ja'd, from Jabir, who said: When the Prophet, peace and blessings of Allah be upon him, would prostrate, he would keep his arms away from his sides until the whiteness of his armpits could be seen.