Sunan an-Nasa'i, Volume 2, Book 12, Hadith 1109 (DarusSalam) ⓘ
It was narrated from 'Ubaidullah bin Abdullah bin Aqram that his father said: "I prayed with the Messenger of Allah and I used to see the whiteness of his armpits when he prostrated."
أَخْبَرَنَا عَلِيُّ بْنُ حُجْرٍ، قَالَ أَنْبَأَنَا إِسْمَاعِيلُ، قَالَ حَدَّثَنَا دَاوُدُ بْنُ قَيْسٍ، عَنْ عُبَيْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ بْنِ أَقْرَمَ، عَنْ أَبِيهِ، قَالَ صَلَّيْتُ مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم فَكُنْتُ أَرَى عُفْرَةَ إِبْطَيْهِ إِذَا سَجَدَ .
also referenced under: Hadith 1108 (Reference) & Book 12, Hadith 80 (In-Book)