Ijtihaad in Islam means striving to understand the shar’i ruling on the basis of shar’i evidence. It is obligatory for the one who is able to do it, because Allaah, may He be glorified and exalted, says (interpretation of the meaning):
“So ask of those who know the Scripture, if you know not”
[al-Nahl 16:43, al-Anbiya’ 21:7].
The one who is able to engage in ijtihaad can find out the truth for himself, but he must have vast knowledge and study the shar’i texts, and understand the guidelines on deriving rulings and be aware of the views of the scholars, lest he fall into that which is contrary to Islam. Some people are seekers of knowledge (taalib al-‘ilm) who have only a little knowledge, but they set themselves up as mujtahids, so you see them acting on the basis of ahaadeeth which are general in meaning but have other reports which make them specific, or they act on the basis of abrogated ahaadeeth and do not know of the texts that abrogate them, or they act on the basis of ahaadeeth which the scholars are unanimously agreed are different from their apparent meanings, but they are unaware of this scholarly consensus.
Such a person is in grave danger. The Mujtahid must have knowledge of the shar’i evidence and knowledge of the basic principles (usool) and scholarly views which, if he knows them, he will be able to derive rulings based on that evidence without unwittingly going against scholarly consensus. If these conditions are met in his case, then he may engage in ijtihaad. Ijtihaad may be focused on a narrow area, so a person may research one issue of knowledge and examine it thoroughly, and become a mujtahid with regard to that issue, or he could focus on one aspect of knowledge, such as issues having to do with tahaarah (purification), which he researches and examines, and thus becomes a mujtahid in that area. End quote.
Fatwa of Shaykh Ibn ‘Uthaymeen, signed by him.