Hadith 2:181
Narrated Ibn Buhainah (RA):
When the Prophet (ﷺ) prostrated while praying, he used to spread out his arms so that the whiteness of his armpits would be visible. [Agreed upon].
وَعَنْ اِبْنِ بُحَيْنَةَ - رضى الله عنه - { أَنَّ اَلنَّبِيَّ - صلى الله عليه وسلم -كَانَ إِذَا صَلَّى فَرَّجَ بَيْنَ يَدَيْهِ , حَتَّى يَبْدُوَ بَيَاضُ إِبِطَيْهِ } مُتَّفَقٌ عَلَيْهِ