Abd al-Rahman bin Yazid (may Allah have mercy on him) said: Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) led the Maghrib prayer, and when he finished the prayer we began looking around. So he said: "What is the matter with you that you are looking around?" We said: We think the sun is still up. So he said: "By the One besides whom there is no god! This is the time of this prayer." Then he recited this verse: "Establish the prayer from the decline of the sun until the darkness of the night" [Surah al-Isra: 78], "This is the decline of the sun, and this is the darkness of the night."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9099