To what extent is the hadith of Umar ibn al-Khattab sound regarding his prohibition of the two types of mut'ah (temporary marriage and tamattu' Hajj) that were practiced during the time of the Messenger of Allah, peace and blessings be upon him, and what are the reasons for the prohibition of mut'ah marriage in the Sunni madhhab?

The Fatwa (definition)'s Summary:

Enjoying a handful (qabdah) of dates and flour was permissible during the time of the Messenger and Abu Bakr, but Umar forbade it because the abrogation had not reached him. This was not an exercise of his own ijtihad (definition), but rather an obedience to the Prophet's command.

As for the mut'ah of Hajj, Umar's prohibition of it was not a proscription, but rather because he chose what was better for people, which was to perform Umrah (definition) and Hajj as two separate journeys. It has been authentically narrated that he said, "If I were to perform Hajj, I would perform mut'ah." He also commanded some people to enter ihram (definition) for both together, emphasizing his adherence to the Prophet's Sunnah (definition). Ibn Umar also used to command mut'ah and clarify that his father's prohibition was not intended as the proscription that some had misunderstood.