When the Prophet ﷺ prostrated, the whiteness of his armpits could be seen.
حدثنا وكيع حدثنا ابن أبي ذئب عن شعبة مولى ابن عباس
أن النبي ﷺ كان إذا سجد يرى بياض إبطيه
Its isnad is hasan. Shubah, the freedman of Ibn Abbas, is Shubah bin Dinar. He is truthful, though there is something to be said about his memory. Ahmad said, "I see no harm in him." Abu Dawud (1:339) narrated another hadith with a different isnad conveying the same meaning from Ibn Abbas. Its meaning is also established in the Sahihayn and other collections through the hadith of Ibn Buhaynah. See al-Muntaqa 961, and it will come at greater length as 2935.