Musnad Ahmad 482:156

Yahya narrated to us, Ibn Lahi'ah informed us, from Bakr ibn Sawadah, from a man, from Rabi'ah ibn Qays, from Uqbah ibn Amir, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "Whoever performs ablution and perfects the ablution, then prays without being heedless or distracted, his preceding sins will be forgiven." And Yahya said once: "Whatever preceded it of evil deeds will be forgiven."