Sunan an-Nasa'i, Book 12, Hadith 119 (In-Book) ⓘ

It was narrated that Maimunah said: "When the Messenger of Allah prostrated, he would hold his arms out to his sides, so that the whiteness of his armpits could be seen from behind. And when he sat he rested on his left thigh."

أَخْبَرَنَا عَبْدُ الرَّحْمَنِ بْنُ إِبْرَاهِيمَ، دُحَيْمٌ قَالَ حَدَّثَنَا مَرْوَانُ بْنُ مُعَاوِيَةَ، قَالَ حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ عَبْدِ اللَّهِ بْنِ الأَصَمِّ، قَالَ حَدَّثَنِي يَزِيدُ بْنُ الأَصَمِّ، عَنْ مَيْمُونَةَ، قَالَتْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم إِذَا سَجَدَ خَوَّى بِيَدَيْهِ حَتَّى يُرَى وَضَحُ إِبْطَيْهِ مِنْ وَرَائِهِ وَإِذَا قَعَدَ اطْمَأَنَّ عَلَى فَخِذِهِ الْيُسْرَى ‏.‏