Severe Warning against Declaring Things Unlawful or Lawful Based on One’s Own Accord
Allaah (The Most High) said:
وَلَا تَقُولُوا لِمَا تَصِفُ أَلْسِنَتُكُمُ الْكَذِبَ هَٰذَا حَلَالٌ وَهَٰذَا حَرَامٌ لِّتَفْتَرُوا عَلَى اللَّهِ الْكَذِبَ ۚ إِنَّ الَّذِينَ يَفْتَرُونَ عَلَى اللَّهِ الْكَذِبَ لَا يُفْلِحُونَ
And say not concerning that which your tongues put forth falsely: “This is lawful and this is forbidden,” so as to invent lies against Allah. Verily, those who invent lies against Allah will never prosper.
Imaam Sadi (rahimahullaah) said: Do not declare (this or that) to be unlawful and lawful on your own accord – lying and fabricating things against Allaah, and forging statements against Him (i.e. passing judgements or rulings that such and such is lawful or unlawful based on ignorance, guessing, assumptions, desires etc).
[إِنَّ الَّذِينَ يَفْتَرُونَ عَلَى اللَّهِ الْكَذِبَ لَا يُفْلِحُونَ – Verily, those who invent lies against Allah will never prosper]; neither (will they proper) in this worldly life nor in the afterlife.
[Source: Tayseer Al-Kareem al-Mannaan Fee Tafseer Kalaam al-Mannaan. Publisher Daar Ibn Hazm 1st Edition 1424AH (Year 2003). Abridged & slightly paraphrased]