Sahih Bukhari
Shortening the Prayers (At-Taqseer)
Volume: 002 / Book Number: 020 / Hadith Number: 209
Narrated 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.