Sunan an-Nasai Book 21, Hadith 1873

أَخْبَرَنَا يُوسُفُ بْنُ حَمَّادٍ، قَالَ حَدَّثَنَا عَبْدُ الْوَارِثِ، عَنْ عَبْدِ الْعَزِيزِ، عَنْ أَنَسٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ مَا مِنْ مُسْلِمٍ يُتَوَفَّى لَهُ ثَلاَثَةٌ مِنَ الْوَلَدِ لَمْ يَبْلُغُوا الْحِنْثَ إِلاَّ أَدْخَلَهُ اللَّهُ الْجَنَّةَ بِفَضْلِ رَحْمَتِهِ إِيَّاهُمْ ‏"‏ ‏.‏

It was narrated that Anas said:"The Messenger of Allah said: 'There is no Muslim, three of whose children die before reaching puberty, but Allah will admit him to Paradise by virtue of His mercy towards them