Musannaf Ibn Abi Shayba 1:1291

'Abdah ibn Sulayman narrated to us, from Sa'id, from Qatadah, from Abu Ja'far, who said: The Prophet (peace and blessings of Allah be upon him) entered upon Umm al-Fadl while Husayn was with her. She handed him to him, and he urinated on his stomach or on his chest. She wanted to take him away from him, but the Prophet (peace and blessings of Allah be upon him) said: "Do not interrupt my son; for the urine of a boy is sprinkled (or splashed) upon, and the urine of a girl is washed."