Sahih Muslim Book 7, Hadith Number 3091.

Chapter : The validity of the Hajj performed by a boy and reward for one who enables him to perform it (by accompanying him and bearing expenses for him).

Ibn Abbas reported that Allah’s Apostle (may peace be upon him) met some riders at al-Rauha and asked who they were. They replied that they were Muslims. They said: Who art thou? He said: (I am) Messenger of Allah. A woman (then) lifted up a boy to him and said: Would this child be credited with having performed the Hajj? Thereupon he said: Yes, and you will have a reward.