Sunan Abu Dawud #495
Sahih (Authentic)
Prayer
مُرُوا أَوْلاَدَكُمْ بِالصَّلاَةِ وَهُمْ أَبْنَاءُ سَبْعِ سِنِينَ وَاضْرِبُوهُمْ عَلَيْهَا وَهُمْ أَبْنَاءُ عَشْرٍ
The Prophet (ﷺ) said: 'Command your children to pray when they become seven years old, and discipline them for it when they become ten.'
Chain of Narration (Isnad)
Narrated 'Amr bin Shu'ayb