Hadhrat Shaykh Abu Ali Daqqaq (May Allah Ta’ala shower His Mercy on him) says: The person who leaves Haraam wealth will be saved from Hell; the person who stays away from doubtful wealth will enter Jannah and the person who saves himself from asking for more will earn proximity (Qurb) of Allah Ta’ala. (the actual words used are wasil ilallah).
Hadhrat Imam Ghazali (May Allah Ta’ala shower His Mercy on him) says: Sin is a tremendous veil. The heart of the person who insists on sins becomes dark and especially Haram earnings will destroy like anything. The Noor created in the heart by Halal earnings is not created by anything else. Thus, a Salik should save himself from Haram earnings and should not eat anything except what is lawful.
Hadhrat Ibrahim bin Adam (May Allah Ta’ala shower His Mercy on him) said that no one achieved any high station with only Salaat, Fasts, Haj, Zakaat and Jihad, only that person achieved greatness and only that person was blessed with Ma'arifah , who earned his living through Halal sources and realized what he eats. This is because with even a mouthful of Haraam, neither the Dua is accepted nor any desire and yearning of worship is developed. Understand it well that the base for every good act is Halal living. Even if you have the privilege of standing up in prayer for the whole night and fasting, even that is beneficial except that what you eat is earned through Halal sources.
Hadith: The Holy Prophet (Sallallahu alaihi wa sallam) says: The person who avoids eating haram food, Allah Ta’ala decides about him that I (i.e. Allah Ta’ala) will feel shy of taking him to account.
And also said: The person who gets tired searching for Halal earnings and comes to his house fully tired, when he sleeps, all his sins are forgiven and when he gets up in the morning, Allah Ta’ala is happy with him.
And said: Until a Muslim does not leave what is not doubtful for the fear of what is doubtful, he will not reach the status of Atqiya (Those who fear Allah Ta’ala)
Hadhrat Abdullah bin Umar (May Allah Ta’ala be well pleased with him) says: Praying after eating Haraam food is like building a house without any foundation, which will not be strong and lasting.
Hadhrat Pirane Pir Shaykh Abdul Qadir Jilani (May Allah Ta’ala shower His Mercy on him) says: Haraam food busies a person in this world ( Dunya ) and creates love for sins. Halal food creates interest in the works of the hereafter Aakhirah and makes one love the obedience of Allah Ta’ala, which earns one the Qurb (closeness) of Allah Ta’ala.
Hadhrat Abdullah bin Abbas (May Allah Ta’ala be well pleased with them) says: If a worshipper (A'abid) worships so much that his back bends like an arch and fasts so much that he becomes thin like an arrow, by Allah Ta’ala, this deed of him will not give any benefit to him until he does not acquire Halal food and truthful speech.
Hadhrat Abu Ali bin Muhammad Rudbari (May Allah Ta’ala shower His Mercy on him): Modesty is the sermonizer (Waiiz) of the heart and modesty before Allah Ta’ala is good than all good things.
Hadhrat Shaykh Abu Ali Daqqaq (May Allah Ta’ala shower His Mercy on him) says: The person who does not speak the truth is like a dumb devil.