How sound is the ḥadīth of Al-Najjād regarding the Prophet's ﷺ combining Maghrib and Isha prayers on a cold, rainy night? And is it established that Abu Bakr, Omar, and Othman combined prayers due to rain?

No authentic hadith has been reported proving that the Prophet, peace and blessings be upon him, combined two prayers due to rain. The hadith of Al-Najjad concerning this matter is very weak.

The hadith of Ibn Abbas in Sahih Muslim indicates that combining prayers due to rain was known during the time of the Prophet, peace and blessings be upon him, as he said: "The Messenger of Allah, peace and blessings be upon him, combined the Dhuhr and Asr prayers, and the Maghrib and Isha prayers in Medina, without fear or rain."

It has been reported from Ibn Umar, may Allah be pleased with him, and a number of the leading Tabi'in in Medina, that they used to combine two prayers on a rainy night. This was a well-known and practiced matter in Medina, and it was not denounced by anyone, which indicates that combining prayers due to rain was customary for them.