It is narrated from Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) used to offer the Maghrib prayer when the sun had set and disappeared behind the veil.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2099