Sunan an-Nasa'i, Volume 2, Book 12, Hadith 1108 (DarusSalam) ⓘ

It was narrated that Abu Hurairah said: "If I were in front of the Messenger of Allah I would be able to see the whiteness of his armpits." (One of the narrators) Abu Mijlaz said: "It is as if he said that because he was praying."

أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ بَزِيعٍ، قَالَ حَدَّثَنَا مُعْتَمِرُ بْنُ سُلَيْمَانَ، عَنْ عِمْرَانَ، عَنْ أَبِي مِجْلَزٍ، عَنْ بَشِيرِ بْنِ نَهِيكٍ، عَنْ أَبِي هُرَيْرَةَ، قَالَ لَوْ كُنْتُ بَيْنَ يَدَىْ رَسُولِ اللَّهِ صلى الله عليه وسلم لأَبْصَرْتُ إِبْطَيْهِ ‏.‏ قَالَ أَبُو مِجْلَزٍ كَأَنَّهُ قَالَ ذَلِكَ لأَنَّهُ فِي صَلاَةٍ ‏.‏