Sahih Muslim Book 8, Hadith Number 3311.

Chapter: It is permissible for the father to give the hand of his daughter in marriage even when she is not fully grown up.

‘Aisha (Allah be pleased with her) reported that Allah’s Apostle (May peace be upon him) married her when she was seven years old, and he was taken to his house as a bride when she was nine, and her dolls were with her; and when he (the Holy Prophet) died she was eighteen years old.