Musannaf Ibn Abi Shayba 35:146

'A'idh ibn Habib narrated to us, from Ash'ath, from Ibn Sirin, who said: "May Allah curse the one who marries a divorced woman to make her lawful for her former husband (al-muhallil) and the one for whom she is made lawful (al-muhallal lahu)." And he mentioned that Abu Hanifa said: "If he marries her to make her lawful for him, then he develops a desire for her, there is no harm in him keeping her."