Sahih Bukhari Volume 003, Book 044, Hadith Number 680.
Narated By 'Abdullah bin Hisham : That his mother Zainab bint Humaid took him to the Prophet and said, "O Allah's Apostle! Take the pledge of allegiance from him." But he said, "He is still too young for the pledge," and passed his hand on his (i.e. 'Abdullah's) head and invoked for Allah's blessing for him. Zuhra bin Ma'bad stated that he used to go with his grandfather, 'Abdullah bin Hisham, to the market to buy foodstuff. Ibn 'Umar and Ibn Az-Zubair would meet him and say to him, "Be our partner, as the Prophet invoked Allah to bless you." So, he would be their partner, and very often he would win a camel's load and send it home.