Prayer (Kitab Al-Salat)
Dawud :: Book 2 : Hadith 497
Narrated Mu'adh ibn Abdullah ibn Khubayb al-Juhani:
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 Apostle of Allah (peace_be_upon_him) was asked about it; he said: When a boy distinguishes right hand from the left hand, then command him to pray.