Majma al-Zawaid — Hadith 4072

Muhammad bin Sirin narrates: I washed Sayyiduna Anas bin Malik (may Allah be pleased with him). When I reached his private parts, I said to his son: "You have more right to wash his private parts; now you take over and wash them." That son began washing him; a cloth was wrapped around his hand, and there was also a cloth over the body of Sayyiduna Anas (may Allah be pleased with him). He washed his private parts from beneath the cloth.
Al-Tabarani recorded it in al-Mu'jam al-Kabir, and its chain is hasan (good).