Musnad Ahmad 33:27

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