Musnad Ahmad 28:123

Hasan narrated to us, Ibn Lahi'ah narrated to us, Huyayy ibn 'Abd Allah narrated to us, that Abu 'Abd al-Rahman narrated to him that he heard 'Abd Allah ibn 'Amr ibn al-'As say: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever goes to the congregational mosque, then one step erases a bad deed, and one step writes for him a good deed, while going and returning."