Sunan an-Nasa'i, Volume 2, Book 12, Hadith 1107 (DarusSalam) ⓘ
It was narrated from Abdullah bin Malik bin Buhainah that: When the Messenger of Allah prayed he held his arms out so much that the whiteness of his armpits appeared.
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 .
also referenced under: Hadith 1106 (Reference) & Book 12, Hadith 78 (In-Book)