عَنِ ابْنِ عَبَّاسٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «أُمِرْتُ أَنْ أَسْجُدَ عَلَى سَبْعَةِ أَعْظُمٍ عَلَى الْجَبْهَةِ وَالْيَدَيْنِ وَالرُّكْبَتَيْنِ وَأَطْرَافِ الْقَدَمَيْنِ وَلَا نَكْفِتَ الثِّيَاب وَلَا الشّعْر»

Maimuna said that when the Prophet prostrated himself he kept his arms so far away from his sides that if a lamb had wanted to pass under his arms it could have done so. This is Abu Dawud’s wording, as [Baghawi] explained in Sharh as-sunna with his isnad. Muslim has something to the same effect

She said that when the Prophet prostrated himself, if a lamb had wanted to pass between his arms it could have done so.