Musnad Ahmad 962:60

Yazid ibn Harun narrated to us, Salim ibn Hayyan informed us, Abu Ghalib narrated to us, he said: I heard Abu Umamah saying: "When you place the purification in its places, you sit having been forgiven. If he stands to pray, it is a virtue and a reward for him, and if he sits, he sits having been forgiven." A man said to him: "O Abu Umamah, do you see if he stands and prays, is it a supererogatory act (nafilah) for him?" He said: "No. The supererogatory act is only for the Prophet, may Allah bless him and grant him peace. How can it be a supererogatory act for him while he is striving in sins and errors? It is a virtue and a reward for him."