Musannaf Ibn Abi Shayba 19:385

Abu Bakr narrated to us, he said: Mu'tamir narrated to us, from Isa al-Khayyat, from al-Sha'bi, who said: Whoever is subject to *li'an* (imprecation) and refuses to swear, the *hadd* (prescribed punishment) is established upon him. And Isa said: I heard al-Sha'bi saying: They are both compelled to perform *li'an* and are imprisoned until they perform *li'an* with each other.