Sayyiduna Abu Mahdhurah (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "When you call the adhan for Maghrib, call it promptly, when the sun has set."
Al-Tabarani recorded it in al-Mu'jam al-Kabir, and its chain is hasan (good).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 1735