Language:
كتاب الأذان
10  
Call to Prayers (Adhaan)
(130)
Chapter: During the prostrations one should keep one's arms away from one's sides and the abdomen should be kept away from the thighs.
(130)
باب يُبْدِي ضَبْعَيْهِ وَيُجَافِي فِي السُّجُودِ
Sahih al-Bukhari 807

Narrated `Abdullah bin Malik bin Buhaina:

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.

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.‏
Reference : Sahih al-Bukhari 807
In-book reference : Book 10, Hadith 202
USC-MSA web (English) reference : Vol. 1, Book 12, Hadith 771
  (deprecated numbering scheme)