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.

أَنَّهُ أَخْبَرَهُ عَنْ مَيْمُونَةَ زَوْجِ النَّبِيِّ ﷺ قَالَتْ كَانَ رَسُولُ اللَّهِ ﷺ إِذَا سَجَدَ خَوَّى بِيَدَيْهِ يَعْنِي جَنَّحَ حَتَّى يُرَى وَضَحُ إِبْطَيْهِ مِنْ وَرَائِهِ وَإِذَا قَعَدَ اطْمَأَنَّ عَلَى فَخِذِهِ الْيُسْرَى

"When the Messenger of Allah ﷺ 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."

كَانَ رَسُولُ اللَّهِ ﷺ إِذَا سَجَدَ خَوَّى بِيَدَيْهِ حَتَّى يُرَى وَضَحُ إِبْطَيْهِ مِنْ وَرَائِهِ وَإِذَا قَعَدَ اطْمَأَنَّ عَلَى فَخِذِهِ الْيُسْرَى

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. (Using translation from Muslim 497a)

عَنْ مَيْمُونَةَ زَوْجِ النَّبِيِّ ﷺ قَالَتْ كَانَ رَسُولُ اللَّهِ ﷺ إِذَا سَجَدَ خَوَّى بِيَدَيْهِ يَعْنِي جَنَّحَ حَتَّى يُرَى وَضَحُ إِبْطَيْهِ مِنْ وَرَائِهِ وَإِذَا قَعَدَ اطْمَأَنَّ عَلَى فَخِذِهِ الْيُسْرَى

"When the Messenger of Allah ﷺ 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." (Using translation from Nasāʾī 1147)

«كَانَ رَسُولُ اللهِ ﷺ إِذَا سَجَدَ خَوَّى بِيَدَيْهِ حَتَّى يُرَى وَضَحُ إِبِطَيْهِ مِنْ وَرَائِهِ وَإِذَا قَعَدَ اطْمَأَنَّ عَلَى فَخِذِهِ الْيُسْرَى»