Is it authentically reported that Umar ibn al-Khattab prohibited mut'ah (temporary marriage), while acknowledging that it is in the Book of Allah and that the Messenger of Allah practiced it?

This hadith has an authentic chain of transmission (sahih isnad). It was narrated by An-Nasa'i from Ibn Abbas, who reported that Umar said: "By Allah, I forbid you from mut'ah, though it is in the Book of Allah, and the Messenger of Allah, peace and blessings be upon him, did it." He was referring to the Mut'ah of Hajj, not Mut'ah of Nikah (temporary marriage). Umar did not intend by this prohibition to contradict the Book and the Sunnah, but rather he exercised ijtihad (independent reasoning) and commanded Ifrad (performing Hajj only) because he saw a benefit for the Muslims in it.