Musnad Abu Ya'la al-Mawsili 152:284

Abu Khaythama narrated to us, Hushaym narrated to us, al-Awwam informed us, from Muhammad ibn Abi Muhammad, the freed slave of Umar ibn al-Khattab, from Abu Ubaydah, 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 for them, except that they will be a fortified fortress for them against the Fire." He said: So we said, "O Messenger of Allah, even if they are two?" He said: "Even if they are two." Abu Dharr said: "O Messenger of Allah, I have not sent ahead except two." He said: "Even if they are two." Ubayy ibn Ka'b, Abu al-Mundhir, the master of the reciters, said: "I have not sent ahead except one." He said: "Even if it is one." He said: "That is only at the first shock."