Is it permissible for a traveller to join two prayers sometimes and not at other times?
Question

Is it permissible for a traveller to join two prayers together sometimes and not join them at other times?.
Praise be to Allah.The Sunnah indicates that it is permissible for a traveller to join prayers. Muslim (705) narrated that Ibn ‘Abbaas (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings of Allah be upon him) joined Zuhr and ‘Asr, and Maghrib and ‘Isha’, in Madeenah when there was no fear and no rain. [According to another report: and no travelling.]: It was said to Ibn ‘Abbaas: Why did he do that? He said: So that his ummah would not be subjected to hardship 
When Ibn ‘Abbaas said that he joined the prayer despite the absence of these three things – travelling, fear and rain – that indicates that they are known reasons for joining prayers. What Ibn ‘Abbaas (may Allah be pleased with him) meant was that the Prophet (peace and blessings of Allah be upon him) joined prayers for a reason other than these three. 
It is permissible for the traveller to join prayers, or to offer each prayer at its own time, as he wishes, but it is better for the traveller not to join prayers unless it will be too difficult for him to offer the prayers at their own times. 
Hence the Prophet (peace and blessings of Allah be upon him) did not join prayers on all his journeys, rather he joined them sometimes and sometimes he did not join them. Some scholars are of the view that it is not permissible for the traveller to join his prayers unless he is on the road, but if he has made a stop then it is not permissible for him to join them. 
But the correct view is that it is permissible for the traveller to join prayers whether he is on the move or has made a stop. It is proven that the Prophet (peace and blessings of Allah be upon him) joined prayers during the campaign to Tabook when he had made a stop. Narrated by Ahmad and Abu Dawood; classed as saheeh by al-Albaani in al-Silsilah al-Saheehah (164). 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) quoted some of the ahadith which indicate that the Prophet (peace and blessings of Allah be upon him) joined prayers whilst travelling when he had made a stop, then he said: 
The apparent meaning of these ahadith is that he used to join prayers when he had made a stop. That was either to show that it is permissible, or because there was a need to join prayers, because the Prophet (peace and blessings of Allah be upon him) did not join his prayers during Hajj when he stopped at Mina. 
Based on that, we say: It is better for the traveller who has made a stop not to join prayers, but if he does join his prayers there is nothing wrong with it. But if he needs to join his prayers, either because he is too tired and needs to rest, or because it is too hard to find water every time, and so on, then it is better for him to join them, and avail himself of the concession. End quote from the essay Mawaaqeet al-Salaah. 
Shaykh Ibn Baaz (may Allah have mercy on him) said:  
The traveller who has made a stop has the choice: if he wishes he may join the prayers at the time of the later prayer or at the time of the earlier prayer. But it is better for him to offer each prayer at its proper time, as the Prophet (peace and blessings of Allah be upon him) did in Mina during the Farewell Hajj. He offered every prayer at its proper time because he was staying there. But if there is a need to join prayers, there is nothing wrong with it, because the Prophet (peace and blessings of Allah be upon him) joined prayers during the campaign to Tabook when he made a stop. End quote. 
Majmoo’ Fatawa Ibn Baz (12/281, 282) 
Based on this, the traveller has the choice between joining prayers or doing each prayer at its own time, but it is better for him not to join prayers unless it is too difficult for him to offer each prayer at its own time. 
But, it should be noted that prayer in congregation is obligatory for the traveller and it is not permissible for him to join prayers and pray alone. 
See also the answer to question no. 40299.