Sa'id bin al-Musayyib (may Allah have mercy on him) said that a Bedouin came to the Prophet (peace and blessings be upon him), pulling out his hair, and said: "O Messenger of Allah! I went in to my wife in Ramadan." So he (peace and blessings be upon him) ordered him to pay the expiation of zihar.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8051