Sayyidah Busrah bint Safwan (may Allah be pleased with her) said: the Messenger of Allah (peace and blessings be upon him) said: "When one of you touches his private parts, let him perform ablution." Urwah said: I objected to this, so Marwan sent a messenger to her, and she narrated this hadith from the Messenger of Allah (peace and blessings be upon him) while I was present.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20876