Yunus said : I once asked Imam Reza (as), I pray to Allah to keep my soul in service for your cause, one of followers had heard that a man gives away sword and horse for the cause of Allah. He went to that man and took the two items but he did not know the rules. His companions met him and told him that working in the way of Allah with these people is not permissible and they commanded them to return the items.
Imam (as) said : He should do so.
The man said : He searched for the man but did not find him. It was said that the man has left.
Imam (as) said : He should serve as a guard but he must not fight.
The man then asked : Should he serve as a guard in Qazwin, al Daylam and Asqalan?
Imam (as) said : No, unless there is fear for the offspring of other Muslims.
The man then asked : Do you say that if Romans entered the lands of the Muslims, they should not stop them?
Imam (as) replied : It is to be on their guard but not fighting. However, if the center of Islam and Muslims is feared for, then one must fight. In such case his fighting is for his own sake and not for the ruler.
The narrator has said that he then asked : If the enemy comes to the place where he serves as a guard then what should he do?
Imam (as) replied : He fights for the center of Islam but not for these people. It is because in the wear and tear of Islam is wear and tear of the religion of Muhammad (saww), O Allah, grant compensation to Muhammad and his family worthy of their services to Your cause.
Ali has narrated from his father from Yahya ibn Abu Imran from Yunus from Imam Reza (as), a similar Hadith.
Divine Treasure 