If a Muslim makes a promise to Allah to stop a particular sin but repeats this sin and therefore breaks his promise to Allah, does the person have to make kafarah: and if so what is it? please give Qur'an or Sunnah as proof.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

If a person breaks his vow to Allah, he should repent again and never despair of the Mercy of Allah. Allah Says (what means): {Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful."} [Quran 39:53]

Having good company (friends), participating in good lessons, and performing as many good deeds as possible are the best ways to purify oneself from sins. Allah Says (what means): {And establish prayer at the two ends of the day and at the approach of the night. Indeed, good deeds do away with misdeeds. That is a reminder for those who remember.} [Quran 11:114]

The Prophet said “Fear Allah wherever you are, and follow up a bad deed with a good deed, it will wipe it out, and behave well towards the people.” [Ahmad, at-Tirmithi and ad-Daarimi]

Allah Says (what means): {Except for those who repent, believe and do righteous work. For them Allah will replace their evil deeds with good. And ever is Allah Forgiving and Merciful.} [Quran 25:70]

So, one should do as many good deeds as possible as these are proofs that one has indeed returned to Allah.

The conditions of sincere repentance are as follows:

1. To give up doing the sin.

2. To resolve oneself never to do it again.

3. To regret having committed it.

4. To repent before one breathes his last. That means one should repent before the stage when the soul is about to leave the body.

5. One should repent in the time allowed for repentance, i.e. before the sun rises from the west.

For more benefit on the expiation for breaking a vow, please refer to Fatwa 102811.

Allah knows best.