Merits of the Helpers in Madinah (Ansaar)
Bukhari :: Book 5 :: Volume 58 :: Hadith 236
Narrated Hisham's father:
Khadija died three years before the Prophet departed to Medina. He stayed there for two years or so and then he married 'Aisha when she was a girl of six years of age, and he consumed that marriage when she was nine years old.