It is narrated from Qadi Shurayh that Sayyiduna Umar bin Al-Khattab, may Allah be pleased with him, wrote to him: If a matter comes to you which is found in the Noble Qur'an, then decide according to it, and do not turn to people's (opinions). And if a matter comes to you which is not in the Book of Allah, then look to the Sunnah of the Messenger of Allah, peace and blessings be upon him, and decide according to it. And if such a matter comes before you in which there is no guidance in the Book of Allah or the Sunnah of the Messenger of Allah, peace and blessings be upon him, then look at the consensus of the people regarding it and adopt that. And if an issue arises which is neither in the Book of Allah, nor in the Sunnah of the Messenger of Allah, peace and blessings be upon him, nor has anyone before you said anything about it, then choose one of two things: if you wish, exercise ijtihad and express your opinion, and if you wish, refrain. And I consider refraining to be better for you.

Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) narrated that when the Prophet (peace and blessings of Allah be upon him) sent him towards Yemen, he asked, "If a case comes before you, how will you decide it?" He replied, "I will decide according to the Book of Allah." The Prophet (peace be upon him) said, "If the matter is not found in the Book of Allah, then what will you do?" He replied, "I will decide according to the Sunnah of the Messenger of Allah (peace be upon him)." He said, "If the matter is not found in the Sunnah of the Messenger of Allah either, then what will you do?" He replied, "I will exercise my judgment with careful thought and will not fall short." The narrator said: The Messenger of Allah (peace be upon him) patted the chest of Sayyiduna Mu'adh (may Allah be pleased with him) (as a gesture of approval) and said, "All praise is due to Allah Who has granted the messenger of the Messenger of Allah the ability to do that which pleases and satisfies the Messenger of Allah."

Harith bin Zuhair said: I think that Abdullah bin Mas'ud (may Allah be pleased with him) said: There was a time upon us when we did not ask any questions, as if we did not even exist in that era, and Allah Almighty decreed that now I have reached the state as you see. So when you are asked about any matter, seek it in the Book of Allah; if you do not find it in the Book of Allah, then seek it in the Sunnah of the Messenger of Allah (peace and blessings be upon him); and if you do not find it in the Sunnah of the Messenger, then look at what the Muslims have agreed upon; and if you do not find the matter in the consensus of the Muslims, then reflect and exercise ijtihad, and do not say, "I fear" or "I am afraid." Surely, the lawful is clear and the unlawful is also clear, and between the two are some doubtful matters, so leave that which is doubtful and adopt that which is certain.

Abdur Rahman bin Yazid also narrated the aforementioned narration from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him).

Qasim bin Abdur Rahman narrated from his father, who narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) in the same manner.

A‘mash (Sulaiman bin Mehran) narrated that Sayyiduna Abdullah bin Mas‘ud (may Allah be pleased with him) said: O people! You will invent new things and new things will be invented for you, so when you see any new matter, hold fast to the original way. The narrator Hafs bin Ghiyath said: Previously, I used to narrate this athar from Habib (bin Thabit) from Abu Abdur Rahman al-Sulami, but then I became doubtful about it, (and he narrated this report from A‘mash from Sayyiduna Abdullah bin Mas‘ud (may Allah be pleased with him), in which there is a break in the chain).

Imam Muhammad bin Sirin, may Allah have mercy on him, said that Sayyiduna Umar, may Allah be pleased with him, said to Sayyiduna Abdullah bin Mas'ud, may Allah be pleased with him: "I have heard that you give fatwas even though you are not the leader. Leave each matter to the one it is suited for."