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.
أَنَّ النَّبِيَّ ﷺ كَانَ إِذَا صَلَّى فَرَّجَ بَيْنَ يَدَيْهِ حَتَّى يَبْدُوَ بَيَاضُ إِبْطَيْهِ وَقَالَ اللَّيْثُ حَدَّثَنِي جَعْفَرُ بْنُ رَبِيعَةَ نَحْوَهُ
Ibn Buhaina, "When the Prophet ﷺ prayed, he used to separate his arms from his body so widely that the whiteness of his armpits was visible."
أَنَّ النَّبِيَّ ﷺ كَانَ إِذَا صَلَّى فَرَّجَ بَيْنَ يَدَيْهِ حَتَّى يَبْدُوَ بَيَاضُ إِبْطَيْهِ وَقَالَ اللَّيْثُ حَدَّثَنِي جَعْفَرُ بْنُ رَبِيعَةَ نَحْوَهُ
When the Prophet ﷺ prostrated, lie spread out his arms so that the whiteness of his armpits was visible.
أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ إِذَا صَلَّى فَرَّجَ بَيْنَ يَدَيْهِ حَتَّى يَبْدُوَ بَيَاضُ إِبْطَيْهِ
When the Messenger of Allah ﷺ prayed he held his arms out so much that the whiteness of his armpits appeared.
أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ إِذَا صَلَّى فَرَّجَ بَيْنَ يَدَيْهِ حَتَّى يَبْدُوَ بَيَاضُ إِبْطَيْهِ
When the Messenger of Allah ﷺ prayed he held his arms out so much that the whiteness of his armpits appeared. (Using translation from Nasāʾī 1106)
أَنَّ رَسُولَ اللهِ ﷺ كَانَ إِذَا صَلَّى فَرَّجَ بَيْنَ يَدَيْهِ حَتَّى يَبْدُوَ بَيَاضُ إِبْطَيْهِ
When the Prophet ﷺ prostrated, lie spread out his arms so that the whiteness of his armpits was visible. (Using translation from Muslim 495a)
أَنَّ رَسُولَ اللهِ ﷺ كَانَ إِذَا صَلَّى فَرَّجَ بَيْنَ يَدَيْهِ حَتَّى يَبْدُو بَيَاضَ إِبِطَيْهِ