وَعَنْ اِبْنِ بُحَيْنَةَ رضى الله عنه
«أَنَّ اَلنَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَانَ إِذَا صَلَّى فَرَّجَ بَيْنَ يَدَيْهِ، حَتَّى يَبْدُوَ بَيَاضُ إِبِطَيْهِ» مُتَّفَقٌ عَلَيْهِ. 11 صحيح
When the Prophet (sallallahu alayhi wa sallam) prostrated while praying, he used to spread out his arms so that the whiteness of his armpits would be visible.
Bulugh al-Maram 360, Book 2, The Book of Prayer, Hadith 181. 1 min read