Sahih al-Bukhari 3564
Sahih al-Bukhari · Book 61: Virtues and Merits of the Prophet (pbuh) and his Companions · The description of the Prophet (saws)
Narrated `Abdullah bin Malik bin Buhaina Al-Asdi:
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ ، حَدَّثَنَا بَكْرُ بْنُ مُضَرَ ، عَنْ جَعْفَرِ بْنِ رَبِيعَةَ ، عَنِ الأَعْرَجِ ، عَنْ عَبْدِ اللَّهِ بْنِ مَالِكٍ ابْنِ بُحَيْنَةَ الأَسْدِيِّ ، قَالَ كَانَ النَّبِيُّ صلى الله عليه وسلم إِذَا سَجَدَ فَرَّجَ بَيْنَ يَدَيْهِ حَتَّى نَرَى إِبْطَيْهِ. قَالَ وَقَالَ ابْنُ بُكَيْرٍ حَدَّثَنَا بَكْرٌ بَيَاضَ إِبْطَيْهِ.
When the Prophet (ﷺ) prostrated, he used to keep his arms so widely apart that we used to see his armpits. (The sub-narrator, Ibn Bukair said, "The whiteness of his armpits.")