It is narrated from Sayyiduna Abu Mahdhurah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said to me: "When you give the adhan for Maghrib, then give it quickly as soon as the sun sets."
Hadith Referenceسلسله احاديث صحيحه / الاذان و الصلاة / 619