Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Answering your question, the late Saudi scholar, Sheikh Ibn Baz (may Allah have mercy on him) stated:
If a menstruating or postpartum bleeding woman becomes pure before sunset, it is obligatory upon her to perform both the Zhuhr and Asr prayers according to the strongest opinion among the scholars.
Similar is the case if she becomes pure before dawn. In that case, she must perform the Maghrib and Isha prayers. This has been narrated from Abdul-Rahman Ibn Awf and Abdullah Ibn Abbas. This is the opinion of the majority of the scholars.
Explaining the reasoning behind this view, Imam Ibn Qudamah states in Al-Moghni that “the time of the second prayer (i.e. Asr and `Isha’) is considered an extension for the first prayer (i.e. Zuhr and Maghrib) for those who have an excuse.”
Let’s clarify that further: a lady whose menses stopped during Asr prayer time had not prayed Zhuhr because she had an excuse (the menses) during the Zhuhr time. But since she became pure during Asr time, she is still in the extension or grace period for Zhuhr prayer and, therefore, she has to pray it along with Asr.
Almighty Allah knows best.
*Source: Islamic Fatawa Regarding Women, Compiled by Muhammad Ibn Abdul-Aziz Al-Musnad and translated by Jamal Al-Din Zarabozo, Darussalam, 1996.