Musnad Ahmad 27:245

Yahya narrated to us, from Ubaydullah, from Nafi’, from Ibn Umar, that the Messenger of Allah (peace and blessings of Allah be upon him) forbade *al-shighar*. I said to Nafi’: "What is *al-shighar*?" He said: "A man marries his daughter to another, and he marries his daughter; and a man marries his sister to another, and he marries his sister, without a dowry."