Truly Allah knows what they hide and what they manifest; surely He does not love the proud. Noble Qur'an (16:23)
Holy prophet says: "He who has a grain of pride in his heart will not enter Paradise." Hence, Pride (Kibr/Takabbur) is a barrier to Paradise.
When a man feels superiority over others and with this a sort of inward elation, this is called pride. Pride man considers himself superior to others for certain qualities of his self. He allots one position to his self and another to others, and then thinks that his position is higher, and is therefore elated. This "puffed up" feeling which imparts a sense of "touch me not" is called pride (Kibr/Takabbur).
A proud man will not tolerate any other to be on equal terms with himself. In private and in public he expects that all should assume a respectful attitude towards him and, acknowledging his superiority, treat him as a higher being. They should greet him first; make way for him wherever he walks; when he speaks everyone should listen to him and never try to oppose him. He is a genius and people are like asses. They should be grateful to him, seeing that he is so condescending.
Virtues are the doors of Paradise, but pride and self-esteem lock them all. So long as man feels elated he will not like for others what he likes for himself. His self-esteem will deprive him of humility, which is the essence of righteousness. He will neither be able to discard enmity and envy, resentment and wrath, slander and scorn, nor will he be able to cultivate truth and sincerity, and calmly listen to any advice. In short, there is no evil which a proud man will not inevitably do in order to preserve his elation and self-esteem.
Being proud about ones own piety is a major sin. This pride can make all the prayers and supplications void. If a person understands the Greatness of Allah (SWT), then he will not be proud and vain at his exertions at prayer. The endowments of the munificent Allah (SWT) are so great that it is impossible to count His Blessings.