Volume 1 · Book 4 · Hadith 225
The Prophet and I walked till we reached the dumps of some people. He stood, as any one of you stands, behind a wall and urinated. I went away, but he beckoned me to come. So I approached him and stood near his back till he finished.
— Sahih Bukhari, Volume 1, Book 4: Ablutions (Wudu'), Hadith #225