The Prophet's (Sallallahu alaihe wasallam) verdict about haraam food
The Prophet (Sallallaho alaihe wasallam) once said:
"As Allah Himself is above all blemishes, likewise, He blesses with His grace only the unblemished things. He enjoins upon the Muslims, what He has laid down for His Prophets. He says in His Holy Book:
"0, Prophets! Eat of the good things and do right. Lo! I am aware of what ye do." (XXIII: 51)
O Ye who believe Eat of the good things where-with we have provided you." (II: 172)
Then the Prophet (Sallallaho alaihe wasallam) did mention of a person who is a way-farer with dishevelled hair and dusty clothes; raising his hands towards the heaven, he calls out: "0, Allah! 0, Allah!" but his food, drink and dress all were from haraam sources. So, Allah would never listen to him and answer his prayers, even though his outward condition showed him deserving.
People wonder why the prayers of the Muslims are not always fulfilled by Allah; the reason is easy enough to understand in the light of the above Hadith.
Though Allah does sometimes grant the prayers of even a Kafir (not to mention the prayer or requests of a sinful Muslim), but it is particularly the prayer of a pious person that is seldom rejected. That is the reason why people generally seek the prayers of such persons for themselves.
It follows that those who wish to have their prayers often granted must abstain from haraam. No wise person would like to run the risk of his prayers being rejected.