Maliks Muwatta Book 29, Hadith Number 5.

Section : The ‘Irrevocable’ Divorce.

Yahya related to me from Malik that he had heard that Umar ibn al-Khattab had heard in a letter from Iraq that a man said to his wife, “Your rein is on your withers (i.e. you have free rein).” Umar ibn al-Khattab wrote to his governor to order the man to come to him at Makka at the time of hajj. While Umar was doing tawaf around the House, a man met him and greeted him. Umar asked him who he was, and he replied that he was the man that he had ordered to be brought to him. Umar said to him, “I ask you by the Lord of this building, what did you mean by your statement, ‘Your rein is on your withers.’?” The man replied, “Had you made me swear by other than this place, I would not have told you the truth. I intended separation by that.” Umar ibn al-Khattab said, “It is what you intended.”