Musnad Ahmad 25:1338

'Abd al-Rahman narrated to us, from Sufyan, from Abu Ishaq, from al-Tamimi, from Ibn 'Abbas, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, would have the whiteness of his armpits visible when he prostrated." Abu 'Abd al-Rahman said: I heard my father say: "Shu'bah used to check on the students of hadith, and one day he said: 'What happened to that handsome young man?'—meaning Shababah."