Sahih Bukhari Volume 008, Book 078, Hadith Number 649.

Narated By Usama : Once a daughter of Allah's Apostle sent a message to Allah's Apostle while Usama, Sa'd, and my father or Ubai were (sitting there) with him. She said, (in the message); My child is going to die; please come to us." Allah's Apostle returned the messenger and told him to convey his greetings to her, and say, "Whatever Allah takes, is for Him and whatever He gives is for Him, and everything with Him has a limited fixed term (in this world): so she should be patient and hope for Allah's reward." Then she again sent for him swearing that he should come; so The Prophet got up, and so did we. When he sat there (at the house of his daughter), the child was brought to him, and he took him into his lap while the child's breath was disturbed in his chest. The eyes of Allah's Apostle started shedding tears. Sa'd said, "What is this, O Allah's Apostle?" The Prophet said, "This is the mercy which Allah has lodged in the hearts of whoever He wants of His slaves, and verily Allah is merciful only to those of His slaves who are merciful (to others)."