Allaah (The Most High) said:
وَمِنۡہُم مَّنۡ عَـٰهَدَ ٱللَّهَ لَٮِٕنۡ ءَاتَٮٰنَا مِن فَضۡلِهِۦ لَنَصَّدَّقَنَّ وَلَنَكُونَنَّ مِنَ ٱلصَّـٰلِحِينَ
فَلَمَّآ ءَاتَٮٰهُم مِّن فَضۡلِهِۦ بَخِلُواْ بِهِۦ وَتَوَلَّواْ وَّهُم مُّعۡرِضُونَ
فَأَعۡقَبَہُمۡ نِفَاقً۬ا فِى قُلُوبِہِمۡ إِلَىٰ يَوۡمِ يَلۡقَوۡنَهُ ۥ بِمَآ أَخۡلَفُواْ ٱللَّهَ مَا وَعَدُوهُ وَبِمَا ڪَانُواْ يَكۡذِبُونَ
”And of them are some who made a covenant with Allah (saying): “If He bestowed on us of His Bounty, we will verily, give Sadaqah (Zakat and voluntary charity in Allah’s Cause) and will be certainly among those who are righteous. Then when He gave them of His Bounty, they became niggardly [refused to pay the Sadaqah (Zakat or voluntary charity)], and turned away, averse. So He punished them by putting hypocrisy into their hearts till the Day whereon they shall meet Him, because they broke that (covenant with Allah) which they had promised Him and because they used to tell lies. [9:75-77]
Imaam Sadi (rahimahullaah) mentioned in his Tafseer that Allaah punished the Munaafiqoon by putting hypocrisy in their hearts; because they made a covenant with Allaah that they will spend in charity if given of Allaah’s Blessings, but they became n*gg*rdly and turned away after they were given of His Blessings.
Therefore, the believer is warned against this evil characteristic of the Munaafiqoon, such that he makes a covenant with his Lord to do such and such, but when he reaches his goal he fails to fulfil that; for indeed, it may be that Allaah will punished him by putting hypocrisy in his heart just as he punished the Munaafiqoon.
[See Tayseerul Kareem Ar-Rahmaan Fee Tafseer Kalaam Al-Mannaan]