Allah, the Blessed and Exalted, says: "And those among you who make zihar from their wives and then go back on what they said, then [upon them is] the freeing of a slave before they touch one another." (to the end of the verse) [Surah al-Mujadilah: 3]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الظهار / Q15255