Musnad Ahmad 1016:7

Qutaybah ibn Sa'id narrated to us, Bakr ibn Mudar narrated to us, from Ja'far ibn Rabi'ah, from al-A'raj, from 'Abdullah ibn Malik ibn Buhaynah, that the Messenger of Allah (peace and blessings of Allah be upon him) used to: "When he prayed, he would spread his arms apart until the whiteness of his armpits appeared."