Hujjiyat ul-Istihsaan (Evidence or proof for its validity as a source of evidence)
The following are from amongst the evidences that they have brought to support Istihsaan as a Hujjah (legal source of proof/evidence):
From the Qur’aan:
The Qawl of Allah Ta’Aalaa:
Those who listen to the speech and follow the best of it (Az-Zumar 18).
And His Qawl Subhaanahu:
And follow the best of what was revealed to you from your Lord (Az-Zumur 55).
In respect to the first Aayah the angle that they have used for proof is that it contains praise and commendation for the one who follows the best speech. The angle of the second Aayah is His Ta’Aalaa’s command to follow the best of what He revealed. If Istihsaan is not a Hujjah (proof) then why does this praise, commendation and matter exist.
As for the Ijmaa’:
Then this is represented in the Ijmaa’ of all of the Fuqahaa in respect to the permissibility to enter the bathroom without specification of the amount of time to be spent in it, the quantity of water used and without specifying the amount that has been paid... This is whilst it is contrary to the conditions and pillars of the ‘Aqd (contract) in Islaam.