Umm Salamah (may Allah be pleased with her), the wife of the Prophet (peace and blessings be upon him), said that the Prophet (peace and blessings be upon him) was sitting, and Hasan and Husayn (may Allah be pleased with them both), or one of them, was sitting in his (peace and blessings be upon him) lap, and he urinated. I stood up to wash it, and the Prophet (peace and blessings be upon him) said: "The urine of a baby boy is to be sprinkled, and the urine of a baby girl is to be washed."
This hadith is authentic from Umm Salamah as her own practice.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4162