During Prostrations one should show his armpits and separate his forearms from his body

Narrated 'Abdullah bin Malik:

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."

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.‏