Musnad Ahmad 482:105

Hashim narrated to us, Layth narrated to us, Yazid ibn Abi Habib narrated to me, from Abu al-Khayr Marthad ibn Abdullah al-Yazani, from Uqbah ibn Amir, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Beware of entering upon women." A man from the Ansar said: "O Messenger of Allah, what do you say about the in-law (al-hamu)?" He said: "The in-law is death."