The Book of The At-Tatbiq (Clasping One's Hands Together)
12/51. Chapter:
Description of prostration
Sunan an-Nasai
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا بَكْرٌ، عَنْ جَعْفَرِ بْنِ رَبِيعَةَ، عَنِ الأَعْرَجِ، عَنْ عَبْدِ اللَّهِ بْنِ مَالِكِ ابْنِ بُحَيْنَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ إِذَا صَلَّى فَرَّجَ بَيْنَ يَدَيْهِ حَتَّى يَبْدُوَ بَيَاضُ إِبْطَيْهِ .
It was narrated from Abdullah bin Malik bin Buhainah that: When the Messenger of Allah (ﷺ) prayed he held his arms out so much that the whiteness of his armpits appeared.
Reference : Sunan an-Nasa'i 1106In-book reference : Book 12, Hadith 78English translation : Vol. 2, Book 12, Hadith 1107