Sahih al-Bukhari 807
Sahih al-Bukhari · Book 10: Call to Prayers (Adhaan) · During the prostrations one should keep one's arms away from one's sides and the abdomen should be kept away from the thighs.
Narrated `Abdullah bin Malik bin Buhaina:
حَدَّثَنَا يَحْيَى بْنُ بُكَيْرٍ ، قَالَ حَدَّثَنِي 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 .
Whenever the Prophet (ﷺ) used to offer prayer he used to keep arms away (from the body) so that the whiteness of his armpits was visible.