Musnad Abu Ya'la al-Mawsili 209:45

Hawthara narrated to us, Mubarak bin Fudala narrated to us, from al-Hasan, from Umm Salama, the wife of the Prophet (may Allah bless him and grant him peace), who said: The Prophet (may Allah bless him and grant him peace) said: "Water is to be poured over the urine of a male infant as long as he has not eaten [solid] food. As for the urine of a female infant, it is to be washed [thoroughly], whether she has eaten [solid] food or not."