The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 1001
This hadith has been narrated by Ja'far b. Rabi' with the same chain of transmitters. And in the narration transmitted by 'Amr b. al-Harith (the words are):" When the Messenger of Allah (rtiay peace be upon him) prostrated, he spread out his arms so that the whiteness of his armpits was visible." And in the narration transmitted by al-Laith (the words are:" When the Messenger of Allah (may peace be upon him) prostrated. he spread his hands from the armpits so that I saw their whiteness."