Sahih Al Bukhari – Book of Shortening The Prayers (At-Taqseer)
Volumn 002, Book 020, Hadith Number 209.
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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.
Junaid Tahir