Sahih Muslim
The Book of Purification (Kitab Al-Taharah)
Book Number: 002 / Hadith Number: 560
A'isha, the wife of the Apostle (may peace be upon him) said: Babies were brought to the Messenger of Allah (may peace be upon him) and he blessed them, and after having chewed (something, e. g. dates or any other sweet thing) he rubbed there with their soft palates. A baby was brought to him and he passed water over him (over his garment), so he asked water to be brought and sprinkled it, but he did not wash it.