Sayyiduna Imran bin Husayn (may Allah be pleased with him) said: The mut'ah (of Hajj) is in the Book of Allah; the Messenger of Allah (peace and blessings be upon him) did not forbid it, nor was any prohibition revealed concerning it. A man said, from his own opinion, as much as he wished. These are the words of Arim's narration. And Ibrahim bin al-Hasan said in his narration only "he did not forbid it", and did not mention the Messenger of Allah (peace and blessings be upon him).
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15334