It is narrated on the authority of Ibn Abbas, that he said that the Messenger of Allah prayed Zuhr and Asr together, and also Maghrib and Isha, although he was neither in a state of fear nor on a journey.

Saheeh Muslim, Vol. I, p.264

Ibn Abbas narrates that the Messenger of Allah recited the Zuhr and Asr prayers together, and also said the Maghrib and Isha prayers,  although he was neither in a state of fear, nor was there any other cause, e.g. rain. It is related from Waki that he asked Ibn Abbas the reason (for the joint prayers). Ibn Abbas replied: So that the followers (of the Prophet) may not experience inconvenience and difficulty.

(1) Saheeh Muslim, Vol. I, p.265
(2) Jami Tirmizee, p. 54  

Abdullah Bin Shaqeeq narrates that one day after the Asr prayer, Ibn Abbas began delivering a sermon which was so long that the sun set and the stars appeared. People began to shout: Salat! Salat! (as it was time for Maghrib prayer). Ibn Abbas paid no heed to them until someone from Bani Tameem stood up and shouted continously: Salat! Salat! Ibn Abbas responded: Woe unto you! You wish to teach me the Sunnat? He said: I have personally seen the Messenger of Allah pray the Zuhr and Asr prayers together, and likewise, the Maghrib and Isha prayers.
 

The narrator says: This statement of Ibn Abbas kept disturbing my conscience until I went and asked Abu Huraira about it, and he attested to what Ibn Abbas had said.

Saheeh Muslim, Vol. 1 p. 265 

It is related from Ibn Abbas that the Prophet at one time recited seven rakats of prayers, and at another time eight rakats, i.e. he recited the Maghrib and Isha prayers together, and the Zuhr and Asr prayers together.

Saheeh Bukhari, Part I, p.72 

It is narrated by Ibn Abbas that in Madina the Prophet used to recite two prayers together (Zuhr and Asr, and Maghrib and Isha), without there being any fear or rainfall. When asked about the reason for this, he said: The Prophet desired that his followers may not suffer inconvenience, and therefore the two prayers may be recited together.