All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Every prayer must be offered in its fixed time as mentioned in the Noble Quran: " When ye pass (Congregational) prayers, celebrate God’s praises, standing, sitting down, or lying down on your sides; but when ye are free from danger, set up Regular Prayers: For such prayers are enjoined on believers at stated times." {An-Nisa`, 103}. In addition, Ibn Masoud (may Allah be pleased with him) asked the Prophet (PBUH): What are the deeds most beloved to Allah? He said: (Prayer at the proper time). He asked: What next? He said: (Being good to your parents). He was asked: What next? He said: (Jihad (striving) in the way of Allah)."{Bukhari}.
Accordingly, it isn`t permissible to either advance or delay prayer without valid excuse. However, Islam has permitted combining Duhr with `Asr and Maghrib with `Isha` in case of travel, rain, and fear. In addition, some scholars permitted this for the sick who faces hardship in offering prayers on their fixed times. They have based this opinion on the following Hadith: Ibn 'Abbas reported: The Messenger of Allah (PBUH) observed the noon and afternoon prayers together, and the sunset and Isha' prayers together without being in a state of fear or in a state of journey." {Related by Muslim}.
"It is permissible for a Muslim to combine Duhr with `Asr at the time of Duhr or at that of `Asr. Thus, it is permissible to combine Jumu`ah with `Asr at the time of Jumu`ah as stated by Al-Zarkashi who adopted this view by analogy to the permissibility of combining Duhr with `Asr due to rain. Rather, combining `Asr with Jumu`ah is more eligible to this concession……….." {Moghni al-Mohtaaj, V.1:P. 529-534}.
In fact, combining prayers is a concession because, in principle, each prayer must be offered in its fixed time. It is well known amongst scholars that concessions aren`t the basis of measure. Thus, it isn`t permissible to measure a new case, which wasn`t stipulated in texts of Sharia, even if it involves greater hardship. For example, since it isn`t permissible to combine prayers due to intense dust and heat, then it isn`t permissible to combine them due to corona virus curfew. This is because the wisdom behind combining prayers is lifting hardship embodied in going back to the mosque to perform the following prayer, and at time of curfew, people are banned from going back to the mosque to offer the following prayer (They are only allowed to go to the mosque to offer Jumu`ah).
Consequently, it isn`t permissible to combine the Jumu`ah and the `Asr prayers when mosques are closed because of the spread of pandemics, as there is no valid excuse permitting such thing. In this situation, a Muslim can offer prayers with his household while at home and will, Inshallah, receive the full reward of praying in the mosque. Rather, our scholars (May Allah have mercy on them) said: "When the household prays in congregation, their reward may be equal to that of congregational prayer in the mosque and even more. {Tohfat Al-Mohtaaj, V.2:P.252}. In some situations, when a Muslim prays alone for a valid excuse, he is given the reward of praying in congregation as mentioned in the following Hadith: Narrated Ibrahim Abu Isma`il As-Saksaki: I heard Abu Burda who accompanied Yazid bin Abi Kabsha on a journey. Yazid used to observe fasting on journeys. Abu Burda said to him, "I heard Abu Musa several times saying that Allah's Apostle said, 'When a slave falls ill or travels, then he will get reward similar to that he gets for good deeds practiced at home when in good health." {Related by Bukhari}. And Allah the Almighty knows best.