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 497 a

Maimuna, the wife of the Messenger of Allah (ﷺ), reported:

When the Messenger of Allah (ﷺ) prostrated himself, he spread his arms, i. e. he separated them so much that the whiteness of his armpits became visible from behind and when he sat (for Jalsa) he rested on his left thigh.
حَدَّثَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ الْحَنْظَلِيُّ، أَخْبَرَنَا مَرْوَانُ بْنُ مُعَاوِيَةَ الْفَزَارِيُّ، قَالَ حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ عَبْدِ اللَّهِ بْنِ الأَصَمِّ، عَنْ يَزِيدَ بْنِ الأَصَمِّ، أَنَّهُ أَخْبَرَهُ عَنْ مَيْمُونَةَ، زَوْجِ النَّبِيِّ صلى الله عليه وسلم قَالَتْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم إِذَا سَجَدَ خَوَّى بِيَدَيْهِ - يَعْنِي جَنَّحَ - حَتَّى يُرَى وَضَحُ إِبْطَيْهِ مِنْ وَرَائِهِ وَإِذَا قَعَدَ اطْمَأَنَّ عَلَى فَخِذِهِ الْيُسْرَى ‏.‏
Reference : Sahih Muslim 497a
In-book reference : Book 4, Hadith 270
USC-MSA web (English) reference : Book 4, Hadith 1003
  (deprecated numbering scheme)