Musnad Ahmad 1165:9

'Affan and Bahz narrated to us, they said: Hammad ibn Salamah narrated to us, he said: 'Ata' al-Khurasani informed us, from Lubabah, Umm al-Fadl, that she was nursing al-Hasan or al-Husayn. She said: The Messenger of Allah (peace and blessings of Allah be upon him) came and lay down in a place that had been sprinkled with water, and she placed him upon his stomach. He urinated on his stomach, and I saw the urine flowing on his stomach. I stood up to get a water skin to pour it over him, but the Messenger of Allah (peace and blessings of Allah be upon him) said: "O Umm al-Fadl, the urine of a boy is to have water poured over it, and the urine of a girl is to be washed." And Bahz said: "Washed." 'Affan narrated to us, he said: Hammad narrated to us, he said: Humayd said: 'Ata' used to narrate it from Abu 'Iyad, from Lubabah.