When and in what context was the noble verse revealed: "and not display their adornment except that which is apparent of it," and was that before the imposition of the niqab?

The scholars have not found a specific determination for the time when the verse of hijab (definition) from Surat An-Nur was revealed. However, Surat An-Nur was revealed due to the incident of the Slander (Al-Ifk) which occurred during the Battle of Al-Muraysi'. Scholars differed on the date of this battle, and Ibn Hajar favored that it was in the fifth year of Hijra. Hijab was made obligatory before that, in the third or fourth year of Hijra, and this is indicated by the hadith (definition) of Aisha regarding the incident of the Slander, and her saying about Safwan, "And he used to see me before the hijab." Verse 31 of Surat An-Nur is considered one of the verses of hijab, and many interpreted the saying of Allah the Almighty: {And let them cast their head-coverings over their bosoms} as obligating the niqab (face-veil), and what is meant by His saying: {except what appears thereof} is clothing and the like. Its reason for revelation, as reported from Muqatil, is that Asma bint Yazid saw women entering upon her without their waist-wraps, so Allah revealed the verse.