Zughban, the freed slave of Habib bin Maslamah (may Allah be pleased with him), said: I saw the Companions of the Messenger of Allah (peace and blessings be upon him) (may Allah be pleased with them), preparing for the two rak'ahs before Maghrib just as they would prepare for the obligatory prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4502