Sahih Muslim Book 2, Hadith Number 563.
Chapter : Pertaining to the urine of the suckling babe, and how it is to be washed away.
Umm Qais daughter of Mihsan reported that she came to the Messenger of Allah (may peace be upon him) with her child, who was not yet weaned, and she placed him in his lap; and he urinated in his (Holy Prophet’s) lap. He (the Holy Prophet) did nothing more than spraying water over it.