one of my friends from Sunni community asked me a question, he said prophet Mohammad (s.a.w) had four daughters and one of them married to umar ibn al khatab.
i don't believe it . what is reality behind this… ?

The question is relating to the daughter of Imam Ali Umm Kulthum

About Umm Kulthum daughter of Imam Ali (as)

There are two traditions (hadiths) narrated by Hisham bin Salim al-Jawaliqi regarding the story of Umm Kulthum’s (peace be upon her) marriage to, neither of which indicates that the marriage has actually occurred.

The first tradition is from Ali bin Ibrahim from his father from Ibn Abi Umayr from Hisham bin Salim and Hammad from Zurarah who narrates that Imam Abi Abdullah al-Sadiq (peace be upon him) said regarding the marriage of Umm Kulthum “That was a Faraj which was taken from us by force” (al-Kafi by al-Kulayni, vol. 5 p. 346).

The phrase ‘Faraj’ was used as a metaphor for the Arabic phrase ‘Irdh’ which means one’s honour (i.e. the direct Irdh for a man is his female relatives such as his mother, wife, sisters, etc.). This has been explained in Bihar ul Anwar by Allamah Majlesi (May God rest his soul in peace).

If we assumed that this tradition was not said based on Taqiyya (the concealment of one’s faith when facing imminent danger to one’s life) then in other words it would mean “our opponents enemies who floated this fabricated story (of the marriage) had taken this Faraj (i.e. Irdh) from us by force”.

The second tradition, with the same Sanad as the previous one, however is from Muhammad ibn Abi Umayr from Hisham bin Salim, who narrates that Imam Abi Abdullah al-Sadiq (peace be upon him) said: “When [Umar] proposed to Amir al-Mu’minin, he [Amir al-Mu’minin] said: ‘She is a child.’

“Then he [Umar] met Abbas and asked him: ‘What is wrong with me? Is there a problem with me?’ Abbas asked: ‘Why?’ Umar replied: ‘I asked your nephew for his daughter’s hand in marriage, and he rejected me. Oh, I swear by Allah, I will fill the well of Zamzam with earth, I will destroy every honour that you have, and I will set up two witnesses to testify that he [Imam Ali peace be upon him] stole, that I may cut off his right hand.’ Abbas thereupon came to Imam Ali (peace be upon him) and informed him of what had transpired. He asked Imam Ali (peace be upon him) to put the matter in his hands, and He complied." (The same previous reference)

If we assumed that this tradition also was not said based on Taqiyya then we would only find that Imam Ali (peace be upon him) let Abbas to deal with Umar’s rudeness and insolence in insisting the marriage (against their will). Meaning that we would not find any proof that the marriage actually occurred.

In fact that what has led some scholars to come to the conclusion that Umar could not get married to Umm Kulthum (peace be upon her) because as Imam Ali (peace be upon him) had put the matter in the hands of Abbas, Abbas delayed the marriage after he had made Umar believe that he would get him married to Umm Kulthum (peace be upon her).

During that time Umar got killed by Abu Lu'lu'ah (May Allah be pleased with him). Meaning that Umar was killed before he could get married to Umm Kulthum (peace be upon her).

This is on the assumption that the previous two traditions were not said based on Taqqiya used for safety reasons; otherwise there is a strong reasonable possibility that they were. For the reason that our Imams (peace be upon them) have advised us that in order for us to ensure whether a particular tradition ,attributed to them, is true or not, we should look it up in the books of Ahlul Khilaf (the opponents to Ahlulbayt). If we find it in their books then we should reject it as it will be either forged or said under Taqiyya (i.e. used to guard themselves against the oppressors).

And as we have originally found the traditions regarding Umm Kulthum’s (peace be upon her) marriage to Umar in the books of Ahlul Khilaf, then according to the advice given by our Imams, it is very likely that those traditions have been falsely attributed to the Imams (peace be upon them) and therefore one may doubt about their validity.

Whatever the case is; even if we assumed that the previous two traditions were not said under Taqiyya, and their interpretation was rather what the opponents of Ahlulbayt claimed it to be true, then they would demonstrate the fact that Umar threatened Amir al-Mu’minin that he would destroy a sign which was appointed by Allah (i.e. fill the well of Zamzam with earth) and cut off the right hand of Amir al-Mu’minin (peace be upon him), due to which Imam Ali (peace be upon him) had to forcibly put the matter in the hands of his uncle Abbas.

They would also demonstrate the fact that Lady Umm Kulthum (peace be upon her) had to forcibly accept that marriage for the sake of religion just how Lady Asiya Bint Muzahim had to sacrifice when she accepted to get married to the tyrant Pharaoh.

To make the answer short most believe that it did not take place and it is a fabrication. Others believe that could have happened but by force.