Whatever was forbidden before the oath is more emphatically forbidden after it. Thus the Companions would pledge allegiance to the Prophet, may Allah bless him and grant him peace, undertaking to obey him and wage jihad alongside him. This was obligatory upon them even without their pledge; the pledge merely reinforced it. No one may break such a covenant.
Likewise, no one may break the pledge of allegiance to the ruler that Allah has commanded must be fulfilled, even without having sworn an oath. How much more so if he has sworn one! Indeed, if a man enters into a contract with another concerning a sale, a lease, or a marriage, he may not betray him and must fulfil that contract. How much more binding, then, is a covenant with those in authority to fulfil what Allah and His Messenger have commanded: obeying them, offering them sincere counsel, and refraining from rebellion against them!
Whenever a contract must be fulfilled without an oath, swearing an oath to fulfil it reinforces it, even if rescinding such a contract is not permissible. Indeed, it is established in an authentic hadith that the Prophet, may Allah bless him and grant him peace, said:
“Whoever possesses four traits is a pure hypocrite, and whoever possesses one of them has a trait of hypocrisy until he abandons it: when he speaks, he lies; when entrusted, he betrays the trust; when he makes a covenant, he breaks it; and when he disputes, he resorts to wickedness.”
Whatever was permissible before an oath does not become forbidden when a man swears an oath concerning it. Rather, he may do it and offer expiation for his oath. Likewise, an act that was not obligatory does not become obligatory upon him when he swears to perform it. Rather, he may offer expiation for his oath and leave it undone, however solemnly he has reinforced that oath and whatever means he has used to reinforce it. People’s oaths do not alter the laws of the religion. No one may, by his oath, forbid what Allah has permitted or make obligatory what Allah has not made obligatory. This is the law revealed to Muhammad, may Allah bless him and grant him peace.