From Sahih Bukhari (Hadith 1107):
Narrated Ibn `Abbas: Allah's Messenger (ﷺ) 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 (ﷺ) used to offer the Maghrib and the `Isha' prayers together on journeys
Reference: Bukhari 1107