In which year was the noble verse that instructs Muslim women to wear the Hijaab revealed?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Ibn Hajar Al-‘Asqalani said in Fat-h Al-Bari that 'the verse of Hijab' as mentioned in some reports regarding the occasions of the revelation refers to the verse mentioned in the question i.e. the verse (which means): {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 Allaah Forgiving and Merciful.} [Quran 33: 59]

He stated in another place that the scholars have three opinions regarding the year in which wearing the Hijab was imposed on women; the third, the fourth, and the fifth year A.H. He adopted the opinion that wearing the Hijab was imposed in the fourth year A.H.

Allaah Knows best.