When does it become Wajib (obligatory) to swear?
It is Wajib to take an oath in a situation where one’s life or honour, or that of another Muslim, is in danger, and taking the oath can ward off the danger. When it is Wajib to protect ones property it is also Wajib to take an oath for its protection. In fact in all the above situations it is Wajib even to take a false oath, although as a precautionary measure one should first try ones best to employ Toriya.