Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "The Messenger of Allah (peace and blessings be upon him) used to combine Maghrib and Isha: he would delay this one (Maghrib) to the end of its time and bring this one (Isha) at the beginning of its time."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9844