Musnad Abu Ya'la al-Mawsili 130:40

Narrated to us ‘Abd al-A‘la, narrated to us Zakariyya ibn Yahya, narrated to us ‘Abd al-‘Aziz ibn Suhayb, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever has three [children] die who have not reached the age of puberty, Allah will enter him into Paradise by the grace of His mercy toward them."