Abd al-Rahman bin Yazid narrated: We were with Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him), and when the sun set he said: "By Allah, besides whom there is no god, this is the time of the sun's decline, and at this very time the time of this prayer has begun."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9102