Musnad Ahmad 37:3

Zayd ibn al-Hubab narrated to us, saying: Abd al-Malik ibn al-Rabi’ ibn Sabrah al-Juhani narrated to me, from his father, from his grandfather, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When a boy reaches seven years of age, he is to be commanded to pray. And when he reaches ten, he is to be struck for it."