Sahih Muslim Book 7, Hadith Number 3092.

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 (Allah be pleased with them) reported: A woman lifted up her child and said: Messenger of Allah, would the child be credited with having performed the Hajj? Thereupon he said: Yes, and there would be a reward for you.