Musnad Ahmad 937:71

Hassan ibn Musa narrated to us, Ibn Lahi'ah narrated to us, from Ubayd Allah ibn Abi Ja'far, that Abu Abd al-Rahman informed him, from Abu Dharr, from the Messenger of Allah (may Allah bless him and grant him peace) that he said: "If a man passes by a door that has no curtain and is not closed, and he looks in, there is no sin upon him. The sin is only upon the people of the house."