From Sahih Muslim (Hadith 5665):
Sayyar reported:I was walking with Thibit al-Bunani that he happened to pass by children and he greeted them. And Thibit reported that he walked with Anas and he happened to pass by children and he greeted them. and Anas reported that he walked with Allah's Apostle (may peace be upon. him) and he happened, to pass by children and he greeted them
Reference: Muslim 5665