Sunan an-Nasa'i 1107

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 ‏.‏

It was narrated that Abu Hurairah said:"If I were in front of the Messenger of Allah (ﷺ) 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

Reference

Sunan an-Nasa'i 1107

In-book

Book 12, Hadith 79