Why is there no room for ijtihad in some issues, such as the prohibition of intoxicants (khamr) and usury (riba)? What is meant by "qat'i al-thubut wa al-dalalah" (definitive in authenticity and meaning)? And what is the difference between these and matters that the Prophet, peace and blessings be upon him, commanded, and the Companions understood as recommended, such as dyeing one's hair (al-ikhtidhab)?

Ijtihad (definition) (independent reasoning) is to be exercised in the absence of explicit textual evidence, and it is not permissible when there is a text that contradicts it. This is because ijtihad and taqlid (definition) (adherence to a school of thought) are rendered invalid when a clear text appears and there is a consensus of scholars on it, due to the Almighty's saying: (And it is not for a believing man or a believing woman, when Allah and His Messenger have decided a matter, that they should have [any] option in their decision). Examples include the prohibition of intoxicants (khamr (definition)) and usury (riba), as their evidence is definitive in authenticity and implication. As for the command concerning dyeing (the hair/beard), it does not contradict this principle, because the default meaning of a command is obligation, unless there is an indication that shifts it from obligation to recommendation or permissibility, such as the command to perform Witr prayer, which other evidence has shifted from obligation.