"O Prophet, tell your wives and your daughters and the women of the believers to bring down over themselves [part] of their outer garments. That is more suitable that they will be known and not be abused. And ever is Allah Forgiving and Merciful." 33:59
And as all Muslims know, The suna ( Prohpet commands) complete the Quran, for example, god asked us to pray 5 times a day right?
but he didn't tell us how and when, but the Prophet PBUH showed us how, same thing goes with the hijab.