Mishkat al-Masabih #530

‘Amr b. Shu'aib said on his father’s authority that his grandfather reported God’s Messenger as saying, “Command your children to observe prayer when they are seven years old, and beat them for sleep together.” Abu Dawud transmitted it; and Baghawi transmitted it thus from him in Sharh as-sunna but in al-Masabih from Sabra b. Ma‘bad.