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.
Hadith Referenceسنن ابن ماجه / كتاب إقامة الصلاة والسنة / 880
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:صحيح مسلم