Musnad Ahmad 26:7

Hashim narrated to us, Al-Awwam informed us, from Muhammad ibn Abi Muhammad, a freed slave of Umar ibn al-Khattab, from Abu Ubaydah ibn Abdullah, from Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There are no two Muslims who have three children die to them, who have not reached the age of puberty, except that they will be a fortified shield for them against the Fire." It was said: "O Messenger of Allah, what if they are two?" He said: "And if they are two." Abu Dharr said: "O Messenger of Allah, I have only sent forth two." He said: "And if they are two." Ubayy ibn Ka'b, Abu al-Mundhir, the master of the reciters, said: "I have only sent forth one." It was said to him: "And if it is one?" He (the Prophet) said: "That is only at the first shock."