Is the hijab an obligation or a Sunnah, and when was it made obligatory upon the believing women?

There is no disagreement among scholars regarding the obligation for a woman to cover her entire body except for the face and hands. As for the face and hands, scholars have differed concerning the obligation to cover them in front of non-mahram (definition) men: the view of Imam Ahmad and the sound opinion within the Shafi'i madhhab (definition) is that it is obligatory to cover them, while Abu Hanifa and Malik hold that it is not obligatory but rather recommended, provided there is no fear of temptation; otherwise, covering becomes obligatory. The hijab (definition) was mandated with the revelation of two verses: 59 from Surat Al-Ahzab and 31 from Surat An-Nur.