It has been narrated on the authority of Umar bin al-Khattab that the Messenger of Allah (ﷺ) said: (The value of) an action depends on the intention behind it. A man will be rewarded only for what he intended. The emigration of one who emigrates for the sake of Allah and His Messenger (ﷺ) is for the sake of Allah and His Messenger (ﷺ) ; and the emigration of one who emigrates for gaining a worldly advantage or for marrying a woman is for what he has emigrated.
It has been narrated through a different chain of transmitters on the authority of Sufyan who said that he heard 'Umar bin al-Khattab relate (this tradition) from the Holy Prophet (ﷺ) while he was delivering a sermon from the pulpit.