It has reached me that whenever the Prophet ﷺ prostrated, the whiteness of his armpits could be seen.
حدثنا عبد الرزاق أخبرنا سفيان عن منصور
بلغني أن النبي ﷺ كان إذا سجد رئي بياض إبطيه
Its isnad is daif due to its being disconnected, for Ibrahim Nakhai belongs to the generation after the followers of the companions, and Imam Ahmad only related it here in order to follow it with a similar hadith from Ibn Abbas.