Musannaf Ibn Abi Shayba 7:387

'Abbad narrated to us, from Sufyan ibn Husayn, who said: I asked al-Hasan and Ibn Sirin about a man who performed *zihar* (declaring his wife unlawful to him like his mother's back) upon his wife and did not offer expiation, taking the matter lightly. They both said: "He should be summoned to the authorities against him."