Musnad Ahmad 26:527

Muhammad ibn Yazid narrated to us, he said: Al-Awwam informed us, Abu Muhammad—a freed slave of Umar ibn al-Khattab—narrated to us, from Abu Ubaydah ibn Abdullah, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) 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 al-Darda’ said: "And two?" He said: "And two." Then Ubayy ibn Ka’b, Abu al-Mundhir, the master of the reciters, said: "And one?" He said: "And one, but that is at the first shock."