It is narrated from Ibn Abbas (may Allah be pleased with them both) that he said: "Shall I not inform you about the prayer of the Messenger of Allah (ﷺ) during travel? When he was at home and the sun had declined, he would combine Dhuhr and Asr at the time of decline. And if he set out on a journey before the sun's decline, he would delay Dhuhr until he combined Dhuhr and Asr at the time of Asr." Kurayb (may Allah have mercy on him) says: According to my understanding, he also said the same regarding Maghrib and Isha.
Hadith Referenceمسند الإمام الشافعي / كتاب الصلاة / 360
Hadith Takhrijأخرجه أحمد: 1/ 367 - وقال الشيخ شعيب الأرنؤوط: صحيح - والدارقطني: 1/ 388، 149 - والبيهقي في معرفة السنن والآثار: 1638 - وقال الألباني: حديث صحيح إرواء الغليل: 3/ 32.