Musannaf Ibn Abi Shayba 1:795

Waki' narrated to us, from Mis'ar, from Uthman ibn Mawhab, regarding a woman who complained to Aisha about the ritual bath (ghusl) from sexual impurity (janabah). She said: "Pour three [handfuls/times], so whatever it reaches, it reaches, and whatever it misses, it misses."