Musnad Ahmad 5:195

Mu'adh ibn Hisham narrated to us, my father narrated to me, from Qatadah, from Abu Harb ibn Abi al-Aswad, from Abu al-Aswad al-Du'ali, from Ali ibn Abi Talib, that the Messenger of Allah (peace and blessings of Allah be upon him) said regarding the infant: "Water is to be sprinkled over the urine of a boy, and the urine of a girl is to be washed." Qatadah said: "This is as long as they have not yet eaten food; but if they have eaten food, both are to be washed."