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 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.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا سُلَيْمَانُ بْنُ دَاوُدَ الْمَهْرِيُّ، حَدَّثَنَا ابْنُ وَهْبٍ، أَخْبَرَنَا هِشَامُ بْنُ سَعْدٍ، حَدَّثَنِي مُعَاذُ بْنُ عَبْدِ اللَّهِ بْنِ خُبَيْبٍ الْجُهَنِيُّ، قَالَ دَخَلْنَا عَلَيْهِ فَقَالَ لاِمْرَأَتِهِ مَتَى يُصَلِّي الصَّبِيُّ فَقَالَتْ كَانَ رَجُلٌ مِنَّا يَذْكُرُ عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم أَنَّهُ سُئِلَ عَنْ ذَلِكَ فَقَالَ " إِذَا عَرَفَ يَمِينَهُ مِنْ شِمَالِهِ فَمُرُوهُ بِالصَّلاَةِ " .
sunnah.com