Hafsah Bint ‘Umar   was born five years before the Messenger of Allah   became a Prophet. She was six years older than her brother 'Abdullaah . She was a beautiful and pious woman. 
Her first marriage was to Khunays Ibn Huthaafah Ibn Qays as-Sahami   who was from Quraysh. He took part with the emigration squads to Abyssinia (Ethiopia) and Madeenah). He fought in the battles of Badr and Uhud. In the latter battle he was fatally wounded and died shortly thereafter in Madeenah. Hafsah   was 18 years old at that time. 
'Umar   was depressed because of his daughter's affliction, since she had become a widow at such a young age. Whenever 'Umar   saw his sad daughter, he grieved so much. After thinking for a long period, he decided to choose a husband for her, to provide her with what she had lost of repose for six months or more.
'Umar   approached Abu Bakr   the dearest person to the Prophet   and offered her hand in marriage to him. 
‘Umar   thought that Abu Bakr   would not refuse to marry a young, pious woman who was the daughter of the man by whom Allah supported Islam. Having sympathetically listened to him, Abu Bakr   did not respond. 
‘Umar   went back with a broken heart and almost did not believe what had happened. He   then went to 'Uthmaan Ibn 'Affaan   whose wife, Ruqayyah   the daughter of the Prophet   had recently died.
‘Umar   offered him his daughter in marriage, but 'Uthmaan   apologized by saying: "I think I would not like to marry at the present time." 
‘Umar's gloom increased by 'Uthmaan's refusal. He became upset and angry with his two companions, mentioning it to the Prophet   complaining about Abu Bakr and ‘Uthmaan . 
Thereupon the Prophet   smiled and said: "Hafsah will get a husband who is better than 'Uthmaan and 'Uthmaan will get a wife better than Hafsah." [Al-Bukhari]
After that, the Prophet   himself married Hafsah    and 'Uthmaan   in turn married Umm Kulthoom   the other daughter of Prophet .
Accordingly, ‘Umar   was elated to receive such a great honor which he had never imagined possible, earlier. He went off to tell the good news to whomever he met. 
When Abu Bakr   met him, he realized the source of his joy. ‘Umar   told him of his unhappiness at Abu Bakr's refusal to reply when he had offered Hafsah's hand to him.
Abu Bakr   congratulated him and apologized by saying: "Do not be angry with me, ‘Umar, the Prophet   mentioned Hafsah before, and I could not disclose the secret of the Prophet . Had he left her, I would have married her." 
Since the Prophet   had mentioned Hafsah to him, he did not want to disclose the Prophet’s   secret.
The people of Madeenah were pleased with the Prophet’s   marriage to Hafsah Bint ‘Umar . It was contracted in the month of Sh'aban in the 3rd year of Hijrah. 
Thus, Hafsah   joined the rank of the pure wives of the Prophet   known as Ummahat Al-Mu'amineen (Mothers of the Believers), who were by then, Sawdah and 'Aa’ishah ; and became a member of the household of the Prophet . 
Sawdah  was happy to see her but 'Aa’ishah, may Allah be please with her, was upset because Hafsah was within her age group and she feared that Hafsah   would share the love of the Prophet . 
‘Umar    knew very well that the Prophet   loved 'Aa’ishah   more than any of the other wives and at times tried to make his daughter aware of this fact. He once said to her: "You are not like 'Aa’ishah and your father is not like her father." 
When the Prophet   deserted his wives because they became too demanding, ‘Umar   once again advised Hafsah   to be careful saying: "You should not be deceived by the one who has been motivated by her beauty and the love of the Prophet   for her (referring to 'Aa’ishah)."
When the Prophet   married his other wives ‘Aa’ishah   began to realize that her protestations were in vain and she made a friend of Hafsah .
Among the other wives of the Prophet   'Aa’ishah   was the most intimate and the closest one to Hafsah . It was Hafsah and 'Aa’ishah   who supported each other against the Prophet   in a domestic intrigue which is mentioned in the Quran. 
It is also mentioned in a tradition reported in the book of Imaam Al-Bukhari : 
'Aa’ishah   narrated: “Allah's Messenger   used to drink honey in the house of Zaynab Bint Jahsh and would stay there with her. So Hafsah and I agreed secretly that if he comes to either of us she would say to him: 'It seems you have eaten Maghaafir (a kind of flower whose nectar is sucked by the honey bee, and has slightly bad smell) for I smell in you the smell of Maghaafir,' (we did so) and he replied: 'No, but I was drinking honey in the house of Zaynab, the daughter of Jahsh, and I shall never take it again. I have taken an oath as to that, and you should not tell anybody about it.’” [Al-Bukhari]
The idea was to suggest to him that this bad smell came from his mouth because of his taking that particular honey.
The Prophet   has always disliked strong smells especially on his breath. He then forbade honey unto himself for which Allah revealed a verse in the Quran and rectified the Prophet . Regarding with this issue Allah Says (what means): "O Prophet, why do you prohibit (yourself from) what Allah has made lawful for you, seeking the approval of your wives? And Allah is Forgiving and Merciful. Allah has already ordained for you (Muslims) the dissolution of your oaths. And Allah is your Protector, and He is The Knowing, The Wise." [Quran 66: 1-2]
Hafsah Bint ‘Umar: The Prophet’s wife in Paradise -II