Sahih Bukhari Volume 008, Book 078, Hadith Number 680.

Narated By 'Umar bin Al-Khattab : I heard Allah's Apostle saying, "The (reward of) deeds, depend upon the intentions and every person will get the reward according to what he has intended. So whoever emigrated for the sake of Allah and His Apostle, then his emigration will be considered to be for Allah and His Apostle, and whoever emigrated for the sake of worldly gain or for a woman to marry, then his emigration will be considered to be for what he emigrated for."