The Prophet ﷺ said, "There are no two Muslim parents who lose three children who have not yet reached the age of accountability, except that Allah will admit them into Paradise through the abundance of His mercy toward them."
حدثنا يوسف بن حماد المعني حدثنا عبد الوارث بن سعيد عن عبد العزيز بن صهيب
عن النبي ﷺ قال ما من مسلمين يتوفى لهما ثلاثة من الولد لم يبلغوا الحنث إلا أدخلهم الله الجنة بفضل رحمة الله إياهم
Bukhari narrated it in the Book of Funerals, in the chapter on the merit of one whose child dies and he seeks reward with Allah for it, hadith no. 1248, from the hadith of Abdul-Warith, with this wording.