Sahih al-Bukhari
Book 10: Call to Prayers (Adhaan)
كتاب الأذان
266 hadith in this book
Chapter 130 During the prostrations one should keep one's arms away from one's sides and the abdomen should be kept away from the thighs.
باب يُبْدِي ضَبْعَيْهِ وَيُجَافِي فِي السُّجُودِ
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.
Hadith text, translations and grading are courtesy of Sunnah.com.