First of all. the general ruling is that prayers must be offered on their fixed times. Therefore, it is not permissible to offer a prescribed prayer before its time or to delay it after its time. This is because Allah the Mighty says: 

Indeed, prayer has been decreed upon the believers a decree of specified times. (An-Nisaa’ 4:103) 

There is a consensus among scholars that it is permissible to combine Maghrib (Sunset Prayer) and `Isha’ (Night Prayer) prayers to be joined at the time of Maghrib, with one adhan (Call to Prayer) and iqamah (call to start the Prayer) for each of them when there is something that causes hardship to people such as heavy rain.

Almighty Allah says:

And has not placed upon you in the religion any difficulty. (Al-Hajj 22:78) 

In another verse, Allah says: 

Allah does not charge a soul except [with that within] its capacity. (Al-Baqarah 2:286) 

To know more about the ruling of combining prayers, watch this short talk with Dr. Muhammad Salah.

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Source: Huda Youtube Channel.