Musnad Abu Ya'la al-Mawsili 155:259

Uqbah narrated to us, Yunus bin Bukayr narrated to us, Yahya bin Ayyub narrated to us, from Abu Zur'ah, from Abu Hurairah, that he narrated to them: That a woman came to the Messenger of Allah, may Allah bless him and grant him peace, with a son of hers who was ill, so that he might pray for his recovery. The Messenger of Allah began to say, "If he dies, Allah will reward you for him." She said, "I have already offered three [children] in Islam."