I live in Britain and I pray in a mosque that is near where I live, but the congregation in this mosque puts Maghrib and ‘Isha’ prayers together at the time of Maghrib. The reason they give for that is that the time period between ‘Isha’ and Fajr prayer is very short and is not sufficient for rest. Please note that the imam of the mosque gives as evidence for that the fact that according to the report of Ibn ‘Abbaas, the Prophet (blessing and peace of Allah be upon him) put together Zuhr and ‘Asr, and Maghrib and ‘Isha’, in Madinah with no excuse.
The shar‘i texts clearly indicate that it is obligatory to offer the five daily prayers at the specified times, and that it is not permissible to put together Zuhr and ‘Asr, and Maghrib and ‘Isha’, without any excuse such as sickness, travelling, rain and the like which would make it difficult to offer each prayer at its own time. Allah, may He be exalted, says (interpretation of the meaning): “Verily, the prayer is enjoined on the believers at fixed hours” [an-Nisa’ 4:103], i.e., the prayer is obligatory at specific and known times.
See: Fath al-Baari by Ibn Rajab al-Hanbali, 3/7-8
Ibn Abi Shaybah (2/346) narrated from Abu Moosa al-Ash‘ari and ‘Umar ibn al-Khattaab (may Allah be pleased with them) that they said: “Putting prayers together without any excuse is a major sin.”
With regard to the report narrated by Muslim (1/489) from Ibn ‘Abbaas (may Allah be pleased with him), who said: The Messenger of Allah (blessings and peace of Allah be upon him) put together Zuhr and ‘Asr, and Maghrib and ‘Isha’, when there was no fear and he was not travelling and it was not raining – this does not mean that he put prayers together without any excuse. Rather it was said to Ibn ‘Abbaas concerning that: What made him do that? He said: He wanted not to make things difficult for his ummah. In other words, he did not want to cause them hardship and difficulty. This indicates that there was an excuse for putting prayers together in this hadeeth, and were it not for putting the prayers together it would have caused hardship for the people.
Al-‘Allaamah Ibn Baaz (may Allah have mercy on him) said in his comment on Fath al-Baari (2/24): The correct way to interpret the hadeeth mentioned is that he (blessings and peace of Allah be upon him) put together the prayers mentioned because of some difficulty that occurred on that day, such as widespread sickness, severe cold or mud and the like. This is indicated by the fact that when he was asked about this putting the prayers together, Ibn ‘Abbaas said: “so that he would not make things difficult for his ummah.” This is a clear answer which indicates that there was an excuse. End quote.
Based on that, quoting this hadeeth as evidence is not appropriate. What is required is to offer each prayer at the time specified for it in sharee‘ah. If there is an excuse or reason for putting prayers together, such as sickness or rain, then there is nothing wrong with putting prayers together in that case.
And Allah knows best.