Musnad Ahmad 27:1365

'Affan narrated to us, Wuhaib narrated to us, Musa ibn 'Uqbah narrated to us, Salim informed me, from his father: "The Messenger of Allah (ﷺ) was approached while he was at the resting place (al-mu'arras) in Dhu al-Hulayfah, in the bottom of the valley. It was said: 'You are in a blessed valley.'"