When the migration took place, Fatima was left in Makka with the rest of the Family which included her step mother Umme Salama, Ali’s(AS)mother Fatima binte Asad and many others. Ali(AS) was in charge of the family. He stayed in Makka for another 3 days to give back the deposits to the Makkans who entrusted these to the Prophet for safe keeping. After fulfilling this duty Ali(AS) brought the family to Madina
After one years stay in Madina when Fatima(SA) was about 18 years old that proposals for marriage began to be received by the Prophet who politely refused to accept by simply saying that it is in the hands of Allah, that he was awaiting Allah’s decree in this matter.
Fatima (SA)was the model of Prophet’s teaching among women just as Ali(AS) was the best embodiment of his instructions and manly qualities among men. They were the most suitable couple to be married. But Ali(AS) was too modest to speak about it. After some persuasion from friends he finally went to see the Prophet in the mosque and proposed for marriage. Prophet told Fatima about it and asked her whether she would approve. After receiving her consent the marriage of Fatima(SA) and Ali(AS) took place in the simplest possible manner. Ali(AS)sold his shield of armour for 200 Dirhams,brought the money to the Holy Prophet who added a similar amount and asked his companions to buy household goods to set up home for the Holy Family.Marriage was solemnized by the Prophet himself and after marriage the couple went to live in a separate house next to the House of the Prophet around the Mosque.
source : http://www.al-islam.org