Sayyiduna Uqbah bin Amir (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Whoever loses three children of his own loins and seeks their reward with Allah" (the narrator Abu Ushshanah would sometimes add the words "in the path of Allah" and sometimes not) "Paradise becomes obligatory for him."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14900