What is the ruling of Islamic law concerning an oath of divorce by three, and is a man considered to have broken his oath if he said to his wife, "I divorce you by three if your sister's husband enters our house," and then this person came and entered the house, standing near the door before leaving immediately?

It is recommended not to get angry. If the husband, by threatening his wife with divorce, merely intended to scare her without making the divorce conditional on her sister's husband entering (the house), then the majority of jurists hold that divorce occurs simply upon entry. Some jurists, however, believe that it does not occur and that he must pay expiation for breaking an oath.

However, if he intended to make her divorce conditional on his (her sister's husband's) entry, then she is divorced immediately upon that. The majority of jurists hold that a triple divorce occurs if it is conditioned on three (declarations), while Shaykh al-Islam Ibn Taymiyyah and Ibn al-Qayyim hold that a single divorce occurs if he did not intend more. In this case, the husband can take her back before the expiry of her 'iddah (waiting period) without a new contract, or mahr (dowry), or a guardian, or witnesses, and it is recommended to have witnesses. It is advisable to consult the Sharia courts.