Musannaf Ibn Abi Shayba 3:1105

Waki’ narrated to us, from Dawud ibn Siwar, from ‘Amr ibn Shu’ayb, from his father, from his grandfather, who said: The Prophet of Allah (may Allah bless him and grant him peace) said: "Command your children to pray when they reach seven [years of age], and strike them for it when they reach ten, and separate them in their beds."