Qiyosning Shariat manbai ekanligiga dalil shuki, ‘illat Shar‘iy matndan olingan yoki unga asoslangan bo‘ladi. Bu Shar‘iy matnlar Qur’on, Sunnat yoki Sahoba ijmosidir. Bu uchtasi shubhasiz manba ekani isbotlangan bo‘lib, ularga hech kim qarshi chiqa olmaydi. Shuning uchun matnda ‘illatning mavjudligi umuman olganda qat’iy (qat’iy dalil) bo‘ladi, qiyos uchun ham xuddi shunday. Matn biror hukmning sababini zikr qilganda, shu hukmni xuddi shu sababga ega bo‘lgan boshqa har qanday masalaga ko‘chirish Ayat yoki Hadis matnining boshqa bir qo‘llanilishi hisoblanadi.

The evidence that Qiyas is a source of Shar'iah is that the „Illah is based upon or derived from a Shari‟ text. These Shari‟ texts are the Quran, the Sunnah, or Ijma' as-Sahabah. These three have been proven as definitive sources of Shari‟ah upon which no one can disagree upon. Therefore, the evidence that the „Illah exists in the text in general is definitive (Qata‟i) and this is also the case for Qiyas. When a text mentions a reason for a rule, extending this rule to any other issue having the same reason is considered another application for the text of the Ayah or Hadith.