Musnad Abu Ya'la al-Mawsili 219:23

Abu Bakr ibn Zanjawayh narrated to us, Abu Mushir narrated to us, he said: Haytham ibn Humayd narrated to me, Al-Ala’ narrated to us, from Makhul, from Anbasah ibn Abi Sufyan, from Umm Habibah, the wife of the Prophet, may Allah bless him and grant him peace, that she heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "Whoever touches his private part, let him perform ablution." Al-Ala’ said: Makhul said: "Whoever touches it intentionally."