1. Unacceptable and Unheard Supplications
Prophet Muhammad said that God revealed to him that He would answer to everyone’s supplications but the one’s who earns by unlawful means.
A man went to Prophet Muhammad and told him that he needed to get an answer to his supplication and asked for a solution. Prophet Muhammad told him that he should be careful that he is using nothing unlawful.
2.Cruelty, as a Result of Unlawful Earning
As mentioned above, Prophet Muhammad said that the supplication of the one who earns unlawful money won’t be answered.
Imam Sadiq also said, “God will never answer to the supplications of a cruel heart.”
From these two sayings, one can infer that there is a link between earning by unlawful means and cruelty of heart.
This is exactly what Imam Hussein said to the army of Kufa on Ashura.
When he realized that they would not understand his words, he told them that they had filled their stomachs with unlawful food and that they had closed their hearts on the truth.
3. Angels Curse Those who Earn by Unlawful Means
Prophet Muhammad said that all the angels on the earth and in the heavens would curse the one who takes a bite out of unlawful food and that they would ask God not to grant this person any bounties as he is surrounded with lawful blessings of God and he uses the unlawful ones.
4. Building a Mosque on a Sabulous Land
God also said in Qur’an, “Which then is best? - he that layeth his foundation on piety to Allah and His good pleasure? - or he that layeth his foundation on an undermined sand-cliff ready to crumble to pieces? and it doth crumble to pieces with him, into the fire of Hell. And Allah guideth not people that do wrong.” (At-Tawba: 109)
5. Being Punished for Good Deeds Based on Unlawful Income
Prophet Muhammad said, “God won’t accept any of the good deeds of those who earn by unlawful means, no matter what that good deed is. They will in fact be punished for these deeds and all that remains for them is what they will take to the Hell with them.”
6. Not a companion of Prophet Muhammad’s Family
Imam Ali said, “The one who devours a believer’s property unjustly does not like us.”
7. No Entrance to Heaven
Prophet Muhammad said, “A body that has been fed by unlawful food is banned to enter to the Heaven.”
source : http://www.tebyan.net