بُلُوغُ الْمَرَامِ
Bulugh al-Maram 298
The Book of Prayer
وَعَنْ اِبْنِ بُحَيْنَةَ - رضى الله عنه - { أَنَّ اَلنَّبِيَّ - صلى الله عليه وسلم -كَانَ إِذَا صَلَّى فَرَّجَ بَيْنَ يَدَيْهِ , حَتَّى يَبْدُوَ بَيَاضُ إِبِطَيْهِ } مُتَّفَقٌ عَلَيْهِ . [1]
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].
Sunnah.com reference: Book 2, Hadith 184