Musnad Ahmad 5:583

'Abd al-Samad ibn 'Abd al-Warith narrated to us, Hisham narrated to us, from Qatadah, from Abu Harb ibn Abi al-Aswad al-Du'ali, [from his father], from 'Ali ibn Abi Talib: That the Messenger of Allah (peace and blessings of Allah be upon him) said: "Regarding the infant: the urine of the boy is to be sprinkled [with water], and the urine of the girl is to be washed." Qatadah said: "And this is as long as they have not yet eaten food; but if they have eaten food, both are to be washed."