Sunan At-Tirmidhi
Hadith #610
Ali bin Abi Talib (may Allah be pleased with him) narrated that :the Messenger of Allah said, about urine of a male child that suckles: ""The urine of the boy is sprinkled, and the girl's urine is washed."" Qatadah (one of the narrators) said: ""This is so, as long as they do not eat, when they eat, then both of them are washed