Sahih Muslim Book 20, Hadith Number 4595.

Chapter : No migration after the conquest of Mecca but allegiance could be sworn (to a rightful leader) on serving the cause of Islam, on fighting in the way of Allah and on following the path of virtue.

It has been reported on the authority of Mujashi’ b. Mas’ud who said: I brought my brother Abu Ma’bad to the Messenger of Allah (may peace he upon him) after the conquest of Mecca and said: Messenger of Allah, allow him to swear his pledge of migration at your hand. He said: The period of migration is over with those who had to do it (and now nobody can get this meritorious distinctions) I said: For what actions will you allow him to bind himself in oath? He said: (He can do so) for serving the cause of Islam, for fighting in the way of Allah and for fighting in the cause of virtue. Abd Uthman said: I met Abd Ma’bad and told him what I had heard from Mujashi’. He said: He has told the truth.