Sahih Muslim
The Book on Salutations and Greetings (Kitab As-Salam)
Book Number: 026 / Hadith Number: 5391
Anas b. Malik reported that when Allah's Messenger (may peace be upon him) happened to pass by young boys he would great them. This hadith has been narrated on the authority of Sayyar with the same chain of transmitters.