It is narrated from Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him) that he said: We used to pray Maghrib with the Messenger of Allah (peace and blessings be upon him) when the sun had hidden behind the veil (of the horizon).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1724