Musnad Ahmad 30:1709

'Arim narrated to us, he said: Mu'tamir narrated to us, he said: And my father narrated to me, from Barakah, from Bashir ibn Nahik, from Abu Hurairah, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to raise his hands in supplication until I could see the whiteness of his armpits." My father said: "And he—meaning the father of Mu'tamir—said: I do not think it was except in the prayer for rain (Istisqa')."