Musnad Ahmad 28:213

Waki’ narrated to us, Suwar bin Dawud narrated to us, from ‘Amr bin Shu’ayb, from his father, from his grandfather, who said: The Messenger of Allah (peace and blessings of Allah be upon him) 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." [‘Abdullah bin Ahmad said]: My father said: "And al-Tufawi, Muhammad bin ‘Abd al-Rahman, said regarding this hadith: 'Suwar is Abu Hamzah,' and he erred in that."