Sahih Bukhari Volume 008, Book 077, Hadith Number 599.

Narated By Usama : Once while I was with the Prophet and Sa'd, Ubai bin Ka'b and Mu'adh were also sitting with him, there came to him a messenger from one of his daughters, telling him that her child was on the verge of death. The Prophet told the messenger to tell her, "It is for Allah what He takes, and it is for Allah what He gives, and everything has its fixed time (limit). So (she should) be patient and look for Allah's reward."