Musannaf Ibn Abi Shayba 9:1753

Hafs narrated to us, from Layth and Mis'ar, from Sa'id ibn Ibrahim, from Humayd ibn 'Abd al-Rahman: He said: 'Umar said: "Beware, let no man enter upon a woman unless she is a mahram (unmarriageable relative) to him." It was said: "What about the brother-in-law (hamw)?" He said: "Beware, the brother-in-law is death."