Sunan Ibn Majah
Hadith #522
It was narrated that Lubabah bint Harith said:""Husain bin 'Ali urinated in the lap of the Prophet and I said: 'O Messenger of Allah, give me your garment and put on another garment.' He said: 'Water should be sprinkled on the urine of a baby boy, and the urine of a baby girl should be washed away