Sunan Ibn Majah, Book 5, Hadith 929 (In-Book) ⓘ
It was narrated from Maimunah that when the Prophet 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.
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 .
also referenced under: Hadith 880 (Reference) & Volume 2, Book 5, Hadith 880 (DarusSalam)