It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that when the Messenger of Allah (peace and blessings be upon him) was in haste to travel, he would delay Zuhr until the time of Asr and combine the two, and he would delay Maghrib and combine Maghrib and Isha when the twilight had disappeared.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5521