It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Ablution is an expiation for the sins before it, and then the prayer takes on the status of nafl."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7539