The believers are those whose hearts tremble (with awe) when Allah is mentioned; whose faith increases when His revelations are recited to them, and who put their trust in their Lord. (al-Qur’an 8:2)
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THE PROPHET 
said: ‘Faith is to believe in Allah, His angels, His Holy Books, His Messengers, the Day of Judgement, and to believe in Divine Destiny – the good and the bad... The highest degree of goodness is to worship Allah as though you see Him, and if you do not see Him, know that He sees you’. (Muslim)
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ABU UMAMAH 
related: ‘A man once asked Allah’s Messenger , ‘What is faith?’ He replied: ‘When your good deeds make you happy and your bad deeds make you sad, then you have faith.’ Then the man asked him, ‘What is sin?’ The Prophet replied: ‘When anything troubles your conscience, stop doing it’. (Ahmad)
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THE PROPHET said: ‘If any of you sees something bad, he should try to change it with his hand; if he cannot do that, then he should change it with his tongue; if he cannot do that, then he should detest it in his heart; and that is the weakest degree of faith’. (Muslim)
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THE PROPHET said: ‘Whoever has three qualities will taste the sweetness of faith: To love Allah and His Messenger more than anything else; to love someone for Allah’s sake alone, and to hate to return to unbelief just as much as one would hate to be thrown into a fire’. (Bukhari, Muslim)