ENOfficial
It was narrated from Maimunah that when the Prophet (ﷺ) prostrated, he would hold his forearms away from his sides, such that if a lamb wanted to pass under his arms, it would be able to do so.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنْ عُبَيْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ بْنِ الأَصَمِّ، عَنْ عَمِّهِ، يَزِيدَ بْنِ الأَصَمِّ عَنْ مَيْمُونَةَ، أَنَّ النَّبِيَّ ـ صلى الله عليه وسلم ـ كَانَ إِذَا سَجَدَ جَافَى يَدَيْهِ فَلَوْ أَنَّ بَهْمَةً أَرَادَتْ أَنْ تَمُرَّ بَيْنَ يَدَيْهِ لَمَرَّتْ .
sunnah.com