The Book of Prayer
Bulugh al-Maram #360
كتاب الصلاة
وَعَنْ اِبْنِ بُحَيْنَةَ - رضى الله عنه - «أَنَّ اَلنَّبِيَّ - صلى الله عليه وسلم -كَانَ إِذَا صَلَّى فَرَّجَ بَيْنَ يَدَيْهِ, حَتَّى يَبْدُوَ بَيَاضُ إِبِطَيْهِ» مُتَّفَقٌ عَلَيْهِ.
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.
وَعَنْ اِبْنِ بُحَيْنَةَ - رضى الله عنه - "أَنَّ اَلنَّبِيَّ - صلى الله عليه وسلم -كَانَ إِذَا صَلَّى فَرَّجَ بَيْنَ يَدَيْهِ , حَتَّى يَبْدُوَ بَيَاضُ إِبِطَيْهِ" مُتَّفَقٌ عَلَيْهِ .
When the Prophet (ﷺ) prostrated while praying, he used to spread out his arms so that the whiteness of his armpits would be visible.
— Bulugh al-Maram, Hadith 360