Sayyidah Maymunah bint al-Harith (may Allah be pleased with her), the wife of the Prophet (peace and blessings be upon him), said: When the Messenger of Allah (peace and blessings be upon him) prostrated, he held his arms so wide apart that a gap could be seen behind him (peace and blessings be upon him), and he kept his armpits open.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20355