Musnad Abu Ya'la al-Mawsili 209:43

Abu al-Rabi' al-Zahrani narrated to us, Ismail—meaning Ibn Ayyash—narrated to us, Ismail ibn Muslim narrated to us, from al-Hasan, from his mother, from Umm Salamah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Water is to be poured over the urine of a male infant, and the urine of a female infant is to be washed."