I heard that Umar(RA) forbid Mutah marriage after the death of the Prophet(PBUH) and Abu Baker. What were his reasons for doing so ?
Looking for answers from Sunni authentic sources.
This is incorrect, and False. Umar never forbade, it was the Prophet (peace and blessings be upon him) who completely and forever forbade it on the day of Khaybar, and this is what authentically known through Ali himself (according to both Sunni and Shia books). It is narrated in both Bukari and Muslim, that Ali said:
نهى رسول الله صلى الله عليه وسلم عن نكاح المتعة يوم خيبر
Muhammad b. 'Ali narrated on the authority of his father 'Ali that Allah's Apostle (ﷺ) on the Day of Khaibar prohibited for ever the contracting of temporary marriage and eating of the flesh of the domestic asses.
List of Ahadith saying that it was prophet Mohammad (pbuh) who banned muta:
sahih Bukhari Volume 5, Book 59, Number 526
sahih Bukhari Volume 5, Book 59, Number 527
Sahih Bukhari Volume 9, Book 86, Number 91
Sahih Muslim Book 008, Number 3260
Sahih Muslim Book 021, Number 4763
Sahih Muslim Book 008, Number 3262
Sahih Muslim Book 008, Number 3263
Sahih Muslim Book 008, Number 3264
Sahih Muslim Book 008, Number 3265
Sahih Muslim Book 008, Number 3266
Sahih Muslim Book 008, Number 3267
There's ample evidence suggesting that it was prophet Mohammad (pbuh) who banned mutah.
Why did Umar reiterate Mohammad (pbuh)'s position on mutah? Because he wanted to enforce the ban.
Why are there contradictory Ahadith on this matter? Let's assume the contradictions weren't resolved during the time of Sahaba, still if we are to read the Quran, it clearly says that mutah was even banned for prophet Mohammad (pbuh):
"It is not lawful for you (to marry other) women after this, nor to change them for other wives even though their beauty attracts you, except those (slaves) whom your right hand possesses. And Allâh is Ever a Watcher over all things." (Qur'an 33:52)
The battle of Khaibar took place two years after the above ayah was revealed, and the list of Ahadith I posted above states clearly that Mohammad (pbuh) banned mutah during the battle of Khaibar, meaning two years after he himself was banned from this practice.
You could argue that the laws which applied to prophet Mohammad (pbuh) does not necessarily apply to us, nor does the laws applied to us necessarily apply to prophet Mohammad (pbuh).
For example we are only allowed to marry 4 wives, but prophet Mohammad (pbuh) was not bound by this law.
The above argument is true, but note as I said before, this matter was resolved by the Sahaba themselves:
"'Ali (Allah be pleased with him) heard that Ibn Abbas (Allah be pleased with them) gave some relaxation in connection with the contracting of temporary marriage, whereupon he said: Don't be hasty (in your religious verdict), Ibn 'Abbas, for Allah's Messenger (may peace be upon him) on the Day of Khaibar prohibited for ever the doing of it-And eating of the flesh of domestic asses." (Muslim 8.3266)
from mutah marriage.
Mutah was halal at the beginning. But then the prophet(PBUH) said it is not permissible after the battle of Kheybar. Thus Umar acting upon this hadith which he heard from the prophet (PBUH) and many hadiths. He forbid mutah marriage when he became caliph:
Ali narrates from the prophet (PBUH):
Messenger (PBUH) prohibited on the Day of Khaibar the contracting of temporary marriage with women and the eating of the flesh of domestic asses.
Answer: At the time of the prophet (PBUH), people didn't have internet nor cellphones to communicate between each other and share hadiths they learned from the prophet . And since some companions at the time of the prophet traveled outside of madinah to other countries, they didn't hear about this prohibition. Thus, when they came back, Umar heard that some of the them wanted to practice mutah marriage. That's when he announced that mutah is haram so that muslims would know about this hadith in case they weren't here.
A narration attributed to Imran ibn Husain reports:
The Verse of Mut'ah was revealed in Allah's Book, so we performed it with Allah's Apostle, and nothing was revealed in Qur'an to make it illegal, nor did the Prophet prohibit it till he died. But the man (who regarded it illegal) just expressed what his own mind suggested. Sahih al-Bukhari, 6:60:43
A narration attributed to Mutarrif reports:
'Imran b. Husain sent for me during his illness of which he died, and said: I am narrating to you some ahadith which may benefit you after me. If I live you conceal (the fact that these have been transmitted by me), and if I die, then you narrate them if you like (and these are): I am blessed, and bear in mind that the Messenger of Allah combined Hajj and Umra. Then no verse was revealed in regard to it in the Book of Allah (which abrogated it) and the Apostle of Allah did not forbid (from doing it). And whatever a person (Umar) said was out of his personal opinion.
Sahih Muslim, 7:2828