Sunan Abi Dawud Book 2, Hadith 106
Waki' misunderstood the name of Dawud b. Sawar. Abu Dawud al-Tayalisi has narrated this tradition from him. He said: Anu Hamzah Sawar al-Sairafi.
The Messenger of Allah (ﷺ) said: Command your children to pray when they become seven years old, and beat them for it (prayer) when they become ten years old; and arrange their beds (to sleep) separately.
Hisham ibn Sa'd reported: We entered upon Mu'adh ibn Abdullah ibn Khubayb al-Juhani. He said to his wife: When (at what age) should a boy pray? She replied: Some person of us reported: The Messenger of Allah (ﷺ) was asked about it; he said: When a boy distinguishes right hand from the left hand, then command him to pray.