Musnad Ahmad 2:269

Abu Abdullah Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from al-Hakam, from 'Umarah ibn 'Umayr, from Ibrahim ibn Abi Musa, from Abu Musa: That he used to issue fatwas for *mut'ah* (temporary marriage/hajj al-tamattu'). A man said to him, "Slow down with some of your fatwas, for you do not know what the Commander of the Faithful has introduced regarding the rites after you." Until he met him (Umar) later and asked him. Umar said, "I know that the Prophet, peace and blessings of Allah be upon him, and his companions did it, but I disliked that they should remain with them (their wives) in the *Arak* (trees/thickets), then depart for Hajj with their heads dripping (from intercourse)."