Musnad Abu Ya'la al-Mawsili 153:46

Abu Khaythama narrated to us, Ahmad ibn Ishaq narrated to us, Wuhayb narrated to us, Musa ibn Uqba narrated to me, from Salim ibn Abdullah ibn Umar, from his father: That the Messenger of Allah (peace and blessings of Allah be upon him) was approached while he was at al-Mu'arras in Dhu al-Hulayfah, in the bottom of the valley. It was said to him: "You are in a blessed valley."