Abd al-Rahman bin Yazid (may Allah have mercy on him) narrates that Abdullah bin Mas'ud (may Allah be pleased with him) used to lead Maghrib while we could see that the sun was about to set. One day we looked towards it, and he said: What are you looking at? They said: We are looking at the sun. Abdullah (may Allah be pleased with him) said: By Allah, besides whom there is no god, this is the time of this prayer. Then he said: "Establish the prayer from the declining of the sun until the darkness of the night" [Surah al-Isra: 78]; "the declining of the sun" (duluk al-shams): "This is the sun's going down."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1730