The Messenger of Allah ﷺ said, "There is no Muslim who loses three children who have not yet reached the age of accountability except that Allah will admit him into Paradise through the abundance of His mercy toward them."
أخبرنا يوسف بن حماد قال حدثنا عبد الوارث عن عبد العزيز
قال رسول الله ﷺ ما من مسلم يتوفى له ثلاثة من الولد لم يبلغوا الحنث إلا أدخله الله الجنة بفضل رحمته إياهم
Bukhari narrated it in Al-Janaiz, chapter on the merit of one whose child dies and he bears it with patience seeking Allah's reward, hadith no. 1248, from the hadith of Abdul-Warith bin Saeed, from Abdul-Aziz bin Suhayb, through the same chain, and it is in Al-Kubra, hadith no. 2001.