Musannaf Ibn Abi Shayba 3:1123

Hushaym narrated to us, from al-‘Awwam, from Ibrahim al-Taymi, who said: They used to consider it desirable to prompt the prayer [to the child], and that he should be taught Arabic. The first thing he should say is: "La ilaha illa Allah" (There is no god but Allah) seven times, so that it is the first thing he speaks.