Allah said,

[وَاعْلَمُواْ أَنَّ اللَّهَ يَحُولُ بَيْنَ الْمَرْءِ وَقَلْبِهِ]

(and know that Allah comes in between a person and his heart.)

Ibn `Abbas commented, "Allah prevents the believer from disbelief and the disbeliever from faith.'' Al-Hakim recorded this in his Mustadrak and said, "It is Sahih and they did not record it.'' . Similar was said by Mujahid, Sa`id, `Ikrimah, Ad-Dahhak, Abu Salih `Atiyyah, Muqatil bin Hayyan and As-Suddi. In another report from Mujahid, he commented;

[يَحُولُ بَيْنَ الْمَرْءِ وَقَلْبِهِ]

(...comes in between a person and his heart.) "Leaves him without comprehension,'' As-Suddi said, "Prevents one self from his own heart, so he will neither believe nor disbelieve except by His leave.'' There are several Hadiths that conform with the meaning of this Ayah. For instance, Imam Ahmad recorded that Anas bin Malik said, "The Prophet used to often say these words,

«يَا مُقَلِّبَ الْقُلُوبِ ثَبِّتْ قَلْبِي عَلَى دِينِك»

(O You Who changes the hearts, make my heart firm on Your religion.) We said, `O Allah's Messenger! We believed in you and in what you brought us. Are you afraid for us' He said,

«نَعَمْ، إِنَّ الْقُلُوبَ بَيْنَ إِصْبَعَيْنِ مِنْ أَصَابِعِ اللهِ تَعَالَى يُقَلِّبُهَا»

(Yes, for the hearts are between two of Allah's Fingers, He changes them (as He wills).)''

This is the same narration recorded by At-Tirmidhi in the Book of Qadar in his Jami' [Sunan], and he said, "Hasan.'' Imam Ahmad recorded that An-Nawwas bin Sam`an Al-Kilabi said that he heard the Prophet saying,

«مَا مِنْ قَلْبٍ إِلَّا وَهُوَ بَيْنَ أُصْبُعَيْنِ مِنْ أَصَابِعِ الرَّحْمَنِ رَبِّ الْعَالَمِينَ إِذَا شَاءَ أَنْ يُقِيمَهُ أَقَامَهُ وَإِذَا شَاءَ أَنْ يُزِيغَهُ أَزَاغَه»

(Every heart is between two of the Fingers of the Most Beneficent (Allah), Lord of all that exists, if He wills, He makes it straight, and if He wills, He makes it stray.)

And he said:

«يَا مُقَلِّبَ الْقُلُوبِ ثَبِّتْ قَلْبِي عَلَى دِينِك»

(O You Who changes the hearts! keep my heart firm on Your religion) And he would say;

«وَالْمِيزَانُ بِيَدِ الرَّحْمنِ يَخْفِضُهُ وَيَرْفَعُه»

(The Balance is in the Hand of Ar-Rahman, He raises and lowers it.)

This was also recorded by An-Nasai and Ibn Majah.

[وَاتَّقُواْ فِتْنَةً لاَّ تُصِيبَنَّ الَّذِينَ ظَلَمُواْ مِنكُمْ خَآصَّةً وَاعْلَمُواْ أَنَّ اللَّهَ شَدِيدُ الْعِقَابِ ]

(25. And fear the Fitnah (affliction and trial) which affects not in particular (only) those of you who do wrong, and know that Allah is severe in punishment.)