Abu Ghalib narrates: I asked Sayyiduna Abu Umamah (may Allah be pleased with him) about the voluntary (nafl) prayer, and he said: "For the Prophet (peace and blessings be upon him) it was nafl (additional), and for you it is a virtue."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 8029