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."  

أَنَّ النَّبِيَّ ﷺ كَانَ إِذَا صَلَّى فَرَّجَ بَيْنَ يَدَيْهِ حَتَّى يَبْدُوَ بَيَاضُ إِبْطَيْهِ وَقَالَ اللَّيْثُ حَدَّثَنِي جَعْفَرُ بْنُ رَبِيعَةَ نَحْوَهُ