Riyad as-Salihin

'Amr bin Shu'aib reported on his father's authority that his grandfather (May Allah be pleased with him) said: Messenger of Allah (ﷺ) said, "Command your children to perform Salat (prayer) when they are seven years old, and beat them for (not offering) it when they are ten, and do not let (boys and girls) sleep together".

[Abu Dawud, who categorized it as Hadith Hasan with a Hasan Chain].