Hijab is obligatory for all Muslim women who have reached the age of puberty.
This general obligation is indicated by the verse in which Allah says (interpretation of the meaning):
“O Prophet! Tell your wives and your daughters and the women of the believers to draw their cloaks (veils) all over their bodies (i.e. screen themselves completely except the eyes or one eye to see the way). That will be better, that they should be known (as free respectable women) so as not to be annoyed. And Allah is Ever Oft-Forgiving, Most Merciful.” [al-Ahzab 33:59]
The women of the Muhajirin and Ansar obeyed that command.
‘Aishah (may Allah be pleased with her) said: “May Allah have mercy on the first Muhajir women. When Allah revealed the words ‘and to draw their veils all over Juyubihinna (i.e. their bodies, faces, necks and bosoms)’ [al-Nur 24:31] they tore their aprons and covered their faces (ikhtamarna) with them.” (Narrated by al-Bukhari, 4480; Abu Dawud, 4102)
What is meant by ikhtamarna is covering the face, as al-Hafiz Ibn Hajar stated in al-Fath, 8/490.
It was narrated that Umm Salamah said: When the words ‘draw their cloaks (veils) all over their bodies’ were revealed, the women of the Ansar went out as if there were crows on their heads because of the way they covered themselves. (Narrated by Abu Dawud, 4101; classed as sahih by Shaykh al-Albani in Sahih Abu Dawud)