Sahih Bukhari Volume 2, Book 20, Hadith Number 209.

Narated By Salim’s father : The Prophet used to offer the Maghrib and Isha’ prayers together whenever he was in a hurry on a journey.

Narrated Ibn Abbas: Allah’s Apostle used to offer the Zuhr and ‘Asr prayers together on journeys, and also used to offer the Maghrib and ‘Isha’ prayers together.

Narrated Anas bin Malik: The Prophet used to offer the Maghrib and the ‘Isha’ prayers together on journeys.