Musnad Ahmad 30:1138

Hashim narrated to us, from Ibn Abi Dhi'b, from al-Aswad ibn al-'Ala' al-Thaqafi, from Abu Salamah, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "From the moment one of you leaves his house for his mosque, one foot records a good deed, and the other erases a bad deed."