Is saying that one must refer only to the Qur’an and Sunnah, and denying consensus (ijma’) and its validity as a proof, on the pretext of there being a Qur’anic verse or an authentic hadith prohibiting something (such as fornication), considered polytheism (shirk) and legislation in place of the Wise Lawgiver?

Shirk in governance and judgment is taking a rival to Allah in ruling and referring to him instead of Allah. The Prophet, peace be upon him, clarified that this includes prohibiting what Allah has made lawful and making lawful what He has prohibited, citing the verse: ﴿They have taken their scholars and monks as lords besides Allah﴾.

The Ahl al-Sunnah (Sunnis) have agreed that the legal proofs are the Quran, the Sunnah, scholarly consensus (Ijma'), and analogical reasoning (Qiyas), and that all of these refer back to the Quran and the Sunnah. For Ijma' and Qiyas are subordinate to them and do not establish independent rulings. The Ummah is infallible from error, and everything upon which it has agreed is truth and in conformity with the Quran and the Sunnah.

Among the benefits of Ijma' are: restricting those who deviate, strengthening trust in the religion and uniting hearts, confirming a ruling by multiplicity of proofs, clarifying a ruling for those for whom the evidence was unclear, reducing disagreement, and censuring the dissenter.