Sayyidah Maymunah (may Allah be pleased with her) said: When the Messenger of Allah (peace and blessings be upon him) prostrated, he kept such a distance (between his arms and body) that the whiteness of his (peace and blessings be upon him) armpits could be seen; he (peace and blessings be upon him) would lean on his right arm and relax the left.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20358