Musnad Abu Ya'la al-Mawsili 152:48

Muhammad narrated to us, Muhammad ibn Zayd al-Wasiti narrated to us, al-Awwam ibn Hawshab narrated to us, from Abu Muhammad, the freed slave of Umar ibn al-Khattab, from Abu Ubaydah, from his father, from the Prophet (may Allah bless him and grant him peace), who said: "Whoever sends ahead three [children] who have not reached the age of puberty, they will be a fortified shield for him against the Fire." Abu Dharr said: "I sent ahead two, O Messenger of Allah." He said: "And two." Ubayy ibn Ka'b, Abu al-Mundhir, the master of the reciters, said: "I sent ahead one, O Messenger of Allah." He said: "And one." He [the Prophet] said: "But that is at the first shock."