Sunan Ibn Majah, Chapter: The Book of the Sunnah

that Yala bin Murrah told them that they had gone out with the Prophet (saw) to a meal to which they had been invited, and Husain was there playing in the street. The Prophet (saw) came in front of the people and stretched out his hands, and the child started to run here and there. The Prophet (saw) made him laugh till he caught him, then he put one hand under his chin and the other on his head and kissed him, and said, "Husain is part of me and I am part of him. May Allah love those who love Husain. Husain is a tribe among tribes." (Hasan) Another chain with similar meaning).