Sahih al-Bukhari 390
Sahih al-Bukhari · Book 8: Prayers (Salat) · During Prostrations one should show his armpits and separate his forearms from his body
Narrated 'Abdullah bin Malik:
أَخْبَرَنَا يَحْيَى بْنُ بُكَيْرٍ ، حَدَّثَنَا بَكْرُ بْنُ مُضَرَ ، عَنْ جَعْفَرٍ ، عَنِ ابْنِ هُرْمُزَ ، عَنْ عَبْدِ اللَّهِ بْنِ مَالِكٍ ابْنِ بُحَيْنَةَ ، أَنَّ النَّبِيَّ صلى الله عليه وسلم كَانَ إِذَا صَلَّى فَرَّجَ بَيْنَ يَدَيْهِ حَتَّى يَبْدُوَ بَيَاضُ إِبْطَيْهِ. وَقَالَ اللَّيْثُ حَدَّثَنِي جَعْفَرُ بْنُ رَبِيعَةَ نَحْوَهُ .
Ibn Buhaina, "When the Prophet (ﷺ) prayed, he used to separate his arms from his body so widely that the whiteness of his armpits was visible."