Sayyiduna Abu Umamah (may Allah be pleased with him) said: (Or he narrated this report:) When you perform ablution properly, you come into a state in which you have been forgiven, and if you offer the prayer, it will be a virtue for you (i.e. a means of earning extra reward). A man said: O Sayyiduna Abu Umamah! What is your view about this: if that person stands and offers the prayer, will it be nafl (supererogatory) for him (i.e. can it be called nafl)? He replied: No! This nafl was for the Prophet (peace and blessings be upon him). How can it be nafl for a man, when a person keeps committing sins and errors? How can it be a virtue and reward for him?
Imam al-Tabarani recorded this report, and its narrators have been declared trustworthy. It is also narrated through another chain, which Imam Ahmad recorded; I have mentioned that report in the chapter "Special characteristics relating to the signs of Prophethood".
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1131