Sayyidah Zaynab bint Jahsh (may Allah be pleased with her) said: The Prophet (peace and blessings be upon him) was taking a midday nap in my house when Sayyiduna Husayn (may Allah be pleased with him), who was still a little boy, came and sat on the stomach of the Messenger of Allah (peace and blessings be upon him), then placed his private part on his (peace and blessings be upon him) navel. She said: So I got up towards him, but he (peace and blessings be upon him) said: "Bring me water." So I brought water, and he (peace and blessings be upon him) poured it over the child, then said: "(The urine of) a girl is washed, and water is poured over (the urine of) a boy."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20518