Hujjar Qiyas a matsayin tushen Shari'a ita ce cewa 'illar (dalilin hukunci) an samo ta ne daga, ko kuma an dogara da ita ga, nassi daga Shari'a. Wadannan nassosi na Shari'a su ne Alkur'ani, Sunna, ko Ijma'in Sahabbai. Wadannan uku an tabbatar da cewa tushen Shari'a ne tabbatattu wanda babu wanda zai sabawa juna a kansu. Saboda haka, hujjar cewa 'illar tana cikin nassi gabaki daya tabbatacciya ce (Qat'i), haka ma Qiyas. Idan wani nassi ya ambaci dalili ga wani hukunci, to mika wannan hukunci zuwa duk wata matsala da ke da dalili iri daya ana ganinta a matsayin wani aiki ko amfani da nassin Ayar ko Hadisin.

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.