Jami` at-Tirmidhi Book 10, Hadith 96
The Messenger of Allah said: "Any Muslim who has lost three of his children will not be touched by the Fire, except for what will fulfill the oath."
"I arrived in Al-Madinah and while I was sitting with Umar bin Al-Khattab they passed by with a funeral, over (a person) whom they were praising with good. Umar said: 'Granted.' I said to Umar: 'What is granted?' He said: 'I said as the Messenger of Allah said: "There is no Muslim about whom three bear witness, except that he is granted Paradise." He said: "And two (as well)." He said: 'We did not ask the Messenger of Allah about one.'"
The Messenger of Allah said: "Whoever has three that precede him (in death) while they did not reach the age of puberty, then they will be a well-fortified fortress for him against the Fire."