كتاب الصلاة
4  
The Book of Prayers
(46)
Chapter: The Description Of The Prayer, With What It Begins And Ends. The Description Of Bowing And Moderation Therein, And Of Prostration And Moderation Therein. Tashah-hud After Each Two Rak'ah Of Four Rak'ah Prayers. Description Ol Sitting Between The Two Prostrations, And In The First Tashah-hud
(46)
باب مَا يَجْمَعُ صِفَةَ الصَّلاَةِ وَمَا يُفْتَتَحُ بِهِ وَيُخْتَمُ بِهِ وَصِفَةَ الرُّكُوعِ وَالاِعْتِدَالِ مِنْهُ وَالسُّجُودِ وَالاِعْتِدَالِ مِنْهُ وَالتَّشَهُّدِ بَعْدَ كُلِّ رَكْعَتَيْنِ مِنَ الرُّبَاعِيَّةِ وَصِفَةَ الْجُلُوسِ بَيْنَ السَّجْدَتَيْنِ وَفِي التَّشَهُّدِ الأَوَّلِ
Sahih Muslim 495 a

'Abdullah b. Malik ibn Bujainah reported:

When the Prophet (ﷺ) prostrated, lie spread out his arms so that the whiteness of his armpits was visible.
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا بَكْرٌ، - وَهُوَ ابْنُ مُضَرَ - عَنْ جَعْفَرِ بْنِ رَبِيعَةَ، عَنِ الأَعْرَجِ، عَنْ عَبْدِ اللَّهِ بْنِ مَالِكٍ ابْنِ بُحَيْنَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ إِذَا صَلَّى فَرَّجَ بَيْنَ يَدَيْهِ حَتَّى يَبْدُوَ بَيَاضُ إِبْطَيْهِ ‏.‏
Reference : Sahih Muslim 495a
In-book reference : Book 4, Hadith 267
USC-MSA web (English) reference : Book 4, Hadith 1000
  (deprecated numbering scheme)