Musnad Ahmad 28:280

Muhammad ibn Abd al-Rahman al-Tafawi and Abdullah ibn Bakr al-Sahmi narrated to us—the meaning is the same—they said: Siwar Abu Hamza narrated to us, from Amr ibn Shu'ayb, from his father, from his grandfather, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Command your children to pray when they are seven years old, and strike them for it when they are ten years old, and separate them in their beds. And if one of you marries off his slave or his employee, he must not look at anything of his 'awrah (private parts), for what is below his navel to his knees is part of his 'awrah."