Sunan Ibn Majah Book 5, Hadith 79
“I was with my father on the plain in
Namirah,*
when some riders passed us and made their camels kneel down
at the
side of the road. My father said to me: ‘Stay with your
lambs until I
go to those people and see what they want.’ He said:
Then he (my
father) went out and I came, (i.e., I came near,) then
there was the
Messenger of Allah (ﷺ), and the time for prayer came
so I prayed
with them, and I was looking at the whiteness of the
armpits of the
Messenger of Allah (ﷺ) every time he prostrated.”
Ibn Majah said: The people say `Ubaidullah bin `Abdullah, but Abu Bakr bin Abu Shaibah said: "The people say `Abdullah bin `Ubaidullah."
Muhammad bin Bashshar said: "`Abdur-Rahman bin Mahdi, Safwan bin `Eisa and Abu Dawud all said: 'Dawud bin Qais narrated to us, from `Ubaidullah bin `Abdullah bin Aqram, from his father, from the Prophet (ﷺ).'" With similar wording.
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.
“I saw the Prophet (ﷺ) when he prostrated and put his knees on the ground before his hands, and when he stood up after prostrating, he took his hands off the ground before his knees.”