Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.117.

Al-Athram records in his Sunnan that Abu Salamah ibn ‘Abdurrahman said: “It is a sunnah to combine the maghrib and ‘isha prayers when it is raining.” Al-Bukhari records that the Prophet sallallahu alehi wasallam combined the maghrib and ‘isha prayers on a rainy night.

A summary of the opinions of the different schools of fiqh on this point follows:

The Shaf’i school says that it is allowed for the resident to combine the zuhr and ‘asr or the maghrib and ‘isha, praying each pair in the time of the earlier salah only, given that it is raining when one begins the earlier prayer and it is still raining by the time one begins the second prayer.

According to the Maliki school, it is allowed to combine the maghrib and ‘isha in the mosque at the time of the maghrib due to rain or expected rain, if there is mud and darkness along the way, or if there is a lot of mud and it prevents the people from wearing their shoes. Nevertheless, he dislikes that the zuhr and ‘asr should be combined due to rain.

According to the Hanbali school, it is only allowed to combine the maghrib and ‘isha in the time of the former or the latter due to snow, ice, mud, severe cold, or rain which soaks the clothes. This concession is allowed only for one who prays with a congregation in the mosque and who comes from a distance over which he could be harmed by the rain. However, for one who prays in a congregation in his house or whose path to the mosque is covered or protected, or for one whose house is right next to the mosque, it is not allowed to combine the salah.