Sahih Bukhari Volume 9, Book 93, Hadith Number 540.

Narrated By Usama : A son of one of the daughters of the Prophet was dying, so she sent a person to call the Prophet. He sent (her a message), “What ever Allah takes is for Him, and whatever He gives is for Him, and everything has a limited fixed term (in this world) so she should be patient and hope for Allah’s reward.” She then sent for him again, swearing that he should come. Allah’s Apostle got up, and so did Mu’adh bin Jabal, Ubai bin Ka’b and ‘Ubada bin As-Samit. When he entered (the house), they gave the child to Allah’s Apostle while its breath was disturbed in his chest. (The sub-narrator said: I think he said, “…as if it was a water skin.”) Allah’s Apostle started weeping whereupon Sa’d bin ‘Ubada said, “Do you weep?” The Prophet said, “Allah is merciful only to those of His slaves who are merciful (to others).”