Sahih Muslim
The Book of Purification (Kitab Al-Taharah)
Book Number: 002 / Hadith Number: 563
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.