Musnad Ahmad 1249:2

Sufyan narrated to us, from Abdullah ibn Abi Bakr ibn Muhammad ibn Amr ibn Hazm, that he heard it from Urwah ibn al-Zubayr while he was with his father narrating, that Marwan informed him, from Busrah bint Safwan, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever touches his private part, let him perform ablution." He said: So he sent a messenger to her while I was present, and she said: "Yes." So he came from her with that.