Breaking of a promise is a very serious sin. It is a sin that sows the seed of hypocrisy in the heart of the person who commits it. The person dies in a state of disbelief and is raised with the hypocrites on the Day of Judgment. The Holy Qur’an says:
“And there are those of them who made a covenant with Allah: If He gives us out of His grace we will certainly give alms and we will certainly be of the good. But when He gave them out of His grace, they become niggardly of it and they turned back and they withdrew. So He made hypocrisy to follow as a consequence into their hearts till the day when they shall meet Him because they failed to perform towards Allah what they had promised Him and because they told lies.” (Surah at-Tawba 9:75-77)
The Shān al-Nuzūl (circumstances of Revelation) of this ayat, according to Tafsīr Minhaj us-Sādiqīn is as follows: “Thulbe Ibn Khatib was a very staunch and a well known Christian. One day he came to the Holy Prophet (S). He began to complain of his poverty, and requested the Holy Prophet (S) to pray for him. The Holy prophet (S) advised him, “Do not emphasise upon this prayer of yours but be patient in your circumstances. Too much of wealth is dangerous for you. If you are grateful for this favour it is better than too much of wealth that makes you unthankful. By Allah if I pray that mountains become gold and silver and move with me, Allah shall cause it to be so but I know that an honourable hereafter is better, and those who are well provided in this world have a difficult hereafter. So, follow the Prophet of Allah and accept his advice.”
But Thulbe the Christian did not follow the advice of the Holy Prophet (S) and again he came back to him with his entreaty and said: “O Prophet of Allah! I make a covenant to Allah that if He gives me abundant wealth I shall spend it upon the deserving ones and do good through it.”
When he persisted in his request, the Holy Prophet (S) prayed for the removal of his property.
Allah (S.w.T.) gave abundant increase in his livestock; and he became a Muslim. His animals multiplied and he became so busy in supervising them that he stopped praying behind the Holy Prophet (S) in the congregational prayers. He prayed only the morning and the evening prayers. Then his herd became so large that the surrounding area of Madinah was not sufficient any more. He took his animals to the outskirts of the town and settled there. Now he was deprived of praying the daily prayers behind the Holy Prophet (S). Still he continued to come to Madinah for the Friday prayers. Then his business expanded beyond the valleys surrounding Madinah. Finally he could not even attend the Friday prayers.
One day the Holy Prophet (S) asked his companions, “Why is Thulbe not attending the congregation?” The people replied, “He has so many goats and sheep that a single valley is unable to accommodate them. Now he has moved to such and such valley and settled there. The Holy Prophet (S) upon hearing this said,
“Woe be upon Thulbe, Woe be upon Thulbe, Woe be upon Thulbe!”
Later when it was Wajib to pay zakat due to the revelation of the verse of Zakat, the Holy Prophet (S) summoned one of his followers and gave him this verse in writing and sent with him a tribesman of Bani Salim. The Prophet ordered, “After you have collected Zakat from Thulbe go to such and such good man and collect it from him too.” Both of them went to Thulbe. They showed him the Qur’anic verse and also gave him the letter of the Holy Prophet (S) containing the rules of Zakat. Thulbe had become so much attached to his wealth that he said, “Muhammad is collecting Jiziya from us! Go somewhere else! Meanwhile I shall think upon it.”
The two men went to this righteous person (of the tribe of Bani Salim). When he saw the ayat and the letter of the Holy Prophet (S), he said, “I hear and I obey the command of Allah and the Prophet.”
He went among his camels and selected the best ones for Zakat and said, “Please take these for the Messenger of Allah (S).” The agents told him that it was not Wajib to give the best of the camels. He said, “How can I not offer the best for Allah and His Prophet?”
Then these two men came back to Thulbe. The wretched fellow repeated his previous objection and refused to pay Zakat.
When the Holy Prophet (S) heard of this incident he said just once, “Woe be upon Thulbe!” Then the Holy Prophet (S) prayed for the well being of the one who had paid the Zakat willingly.
The companions used to wonder why Thulbe had not accepted the advice of the Holy Prophet (S) and saved himself from apostasy! (Zakat is one of the Dhururiyāt al-Dīn and one who says that Zakat is not Wajib has apositisised and is no more a Muslim!) The ayats quoted before this discussion were revealed for Thulbe.