Valid custom approved by the Shari'ah is one of the evidences and tools for deducing Islamic rulings. In some cases, it is the most important and sometimes the only document for responding to the daily needs of society in the field of ijtihad (jurisprudential reasoning). The Shari'ah has approved the custom of custom in non-worship matters. Since rational presumptions do not need to be authorized by the Shari'ah, their rationality suffices for their authenticity. This is because the audience of religious rulings are the common people, and the religious orders have been established for them to live better lives. Entrusting the diagnosis of issues to specialized customs is effective in many cases and leads to uniformity in the construction of logic. The custom in question is a precise and tolerant custom that is carefully adapted and recognized. In this case, there is no prohibition in the Shari'ah. It cannot be said that a custom with this characteristic, which is a product of rational nature and as a general theorem of proof, is a proof in the time of the presence of the Infallibles (peace be upon them) and not a proof in the age of absence. On this basis, custom is one of the evidences and tools that can be used to deduce Islamic rulings.