Musnad Abu Ya'la al-Mawsili 155:229

Qasim ibn Abi Shaybah narrated to us, Hafs narrated to us, from Abd al-Rahman ibn Ishaq, from Yazid ibn al-Hakam, from Uthman ibn Abi al-Aas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, he has shielded himself with a fortified shield, whoever has had three children pass away before him in Islam."