Musnad Ahmad 27:1148

He said: And Nafi‘ said: That ‘Abdullah ibn ‘Umar narrated to him, "That the Messenger of Allah (ﷺ) used to descend beneath a large *sarhah* (acacia) tree below al-Ruwaythah, to the right of the road, in a flat, level place, when emerging from the hillock, two miles before the *barid* (post-station) of al-Ruwaythah. Its top had broken off, and it was standing on a trunk."