Musar for Sanhedrin 161:6

For it is not only the explicit and well known deed of theft or oppression which is forbidden but rather anything which may eventually lead to such a deed and cause it is included in the prohibition. On this our sages of blessed memory said (Sanhedrin 81a): "'he did not defile the wife of his fellow' (Yechezkel 18:6) - that he did not encroach upon his fellow's trade".