It is narrated from Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Pray the Maghrib prayer as soon as the sun sets; perform it before the star appears."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 4043