His sermon on Temporary marriage and mutat al-hajj.
Useful information: The Sermon of 'Umar
In a famous sermon the second caliph 'Umar banned mut'a with the following words: 'Two mut'a were practiced during the time of the Prophet [i.e. temporary marriage and mutat al-hajj], but I forbid both of them and will punish anyone who practices either.' Al-Razi summarizes the Sunni interpretation of 'Umar's words by saying that they were pronounced in a gathering of Companions and no one protested. Therefore, the situation must have been as follows: either (1) everyone knew that mut'a was forbidden, so they remained silent; or (2) they all knew that it was permitted, yet they remained silent out of negligence and in order to placate 'Umar; or (3) they did not know whether it was forbidden or permitted, so they remained silent since the matter had just then been clarified for them, so they had no reason to protest.