Musannaf Ibn Abi Shayba 9:1179

Yazid ibn Harun narrated to us, from Sa'id ibn Abi 'Aruba, from Ma'mar, from al-Zuhri, from 'Abd al-Malik ibn al-Mughira ibn Nawfal, that Ibn 'Umar was asked about a man making his wife lawful [for her previous husband]. He said: "That is fornication (al-sifah). If 'Umar were to catch you, he would have bereaved you [of your lives/he would have punished you severely]."