Musnad Ahmad 2:260

'Abd al-Razzaq narrated to us. He said: And Hushaym informed me, from al-Hajjaj ibn Artah, from al-Hakam ibn Utaybah, from 'Amarah, from Abu Burdah, from Abu Musa: That 'Umar said: "It is the Sunnah of the Messenger of Allah, may Allah bless him and grant him peace"—meaning the Mut'ah (temporary marriage). "But I fear that they will consummate the marriage with them under the Arak trees, then depart with them as pilgrims."