Sahih Bukhari, Chapter: Sales and Trade

Once the Prophet went out during the day. Neither did he talk to me nor I to him till he reached the market of Bani Qainuqa and then he sat in the compound of Fatima's house and asked about the small boy (his grandson Al-Hasan) but Fatima kept the boy in for a while. I thought she was either changing his clothes or giving the boy a bath. After a while the boy came out running and the Prophet embraced and kissed him and then said, 'O Allah! Love him, and love whoever loves him.'