The Rules of the Oath
Issue 943: When someone swears an oath (Yameen/Qasam) it is obligatory that he act according to this oath when the following conditions are present, otherwise, the atonement (Kaffaarah) is obligatory upon him.
1) That the one swearing be sane and mature. When the person is swearing an oath regarding his property it is conditional that he not be an incompetent (Safeeh) or prevented from disbursing his property by the Religious Authority (Al-Haakim Ash-Shar') and that his swearing be with proper intention and free will. Then, the oath of a child, the insane, the compelled and the one financially restricted (in utilizing his property) is not proper. Likewise, the oath of the one swearing in the state of anger when he swears without proper intention and volition.
2) The action which is sworn to be established is conditional that it not be unlawful nor disliked (Makrooh) or that his abandoning something is not something that is obligatory nor recommended. When the oath was to perform a permissible (Mubaah) act, it is obligatory that it not be of those acts whose abandonment is better than its performance in the view of people. Likewise, when the oath is to abandon a permissible (Mubaah) act, it is obligatory that it not be (of those acts whose) performance is better than is abandonment in the view of people.
3) When someone swears with the name of Allah be He Exalted, whether it is one of which is not applied to another besides Allah, like the word Allah or whatever is its equivalence in another language. Or it is a name which is applied to other than Allah also, however, it is known from context that his intention is Allah, rather, when he swears by the names from which Allah the Exalted is not understood without an indication, but he intended Allah the Exalted, then, the obligatory precaution to abide by his oath.
4) (It is conditional) that the oath be executed by the one swearing's tongue. Then, it does not suffice when he repeated it (only) in his heart. The precaution in writing (an oath) is to act in accordance with it. However, the mute person when he swears by gestures, it is proper.
5) The action sworn to be possible. When, at the time of swearing it was possible, but he became unable (to perform it) afterward or there is great difficulty in (performing it), his oath will be invalidated from the time when the new situation overtakes him (meaning the inability befalls him or the difficulty).