Sunan Abu Dawud #495

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.'