Musnad Ahmad 25:922

Muhammad ibn Abdullah ibn al-Zubayr narrated to us, Isra'il narrated to us, from Abu Ishaq, from al-Tamimi, from Ibn Abbas, who said: "I contemplated the prayer of the Messenger of Allah, peace and blessings of Allah be upon him, and I saw him performing *mukhawwi* (keeping his arms away from his sides), and I saw the whiteness of his armpits."