The fulfillment of promise is a virtue, which Islam recommends. The breaking of promise is a vice, which it prohibits.
Allah the Most High said: “O you who believe, why you say that which you do not do? It is most hateful to Allah that you should say that which you do not do.”
The Holy Prophet Muhammad (S) said: “He who believes in Allah and the Day of Judgement must fulfill his promises.”
He (S) also said: “He who has these three things should he counted a hypocrite, even if he performs the daily prayers, observes fasting and thinks that he is a Muslim: breach of trust, lying and breach of promise.”
He (S) also said: “Anyone who does not wrong people in his behaviour towards them, does not lie in his talks with them, and does not break his promises to them; he is among those whose manliness is complete, whose justice is manifest, whose brotherhood is indispensable, and whose backbiting is forbidden.”
Imam ‘Ali (a.s.) the Commander of the Faithful said: “Breach of promise brings the hatred of Allah and of people. Allah says: ‘It is most hateful to Allah that you should say that which you do not do.’1”
Imam Ja’far al-Sadiq (a.s.) said: “A promise which a believer makes to his brother is a vow that has no expiation; so he who fails to fulfill it, his failure will he counted against Allah and he will he exposed to His hatred. Allah says: ‘O you who believe, why do you say that which you do not do? It is most hateful to Allah that you should say that which you do not do.’2”
He (a.s.) also said: “A believer is a brother to his believing fellow, an eye and a guide for him. He never betrays wrongs or tricks him, nor does he make to him a promise which he breaks.”