Fulfilling promises is a noble trait emphasized by the Qur'an and Sunnah. Islam considers it a mark of a truthful and sincere believer. Breaking a promise, especially knowingly, is a form of dishonesty and one of the signs of hypocrisy.
Allah says:
وَأَوْفُوا۟ بِٱلْعَهْدِ ۖ إِنَّ ٱلْعَهْدَ كَانَ مَسْـُٔولًۭا
"And fulfill [every] commitment. Indeed, the commitment is ever [that about which one will be] questioned."
— Surah Al-Isrā' (17:34)
The Prophet ﷺ said:
"آيَةُ الْمُنَافِقِ ثَلَاثٌ: إِذَا حَدَّثَ كَذَبَ، وَإِذَا وَعَدَ أَخْلَفَ، وَإِذَا اؤْتُمِنَ خَانَ"
"The signs of a hypocrite are three: when he speaks, he lies; when he promises, he breaks it; and when he is entrusted, he betrays."
— Sahih al-Bukhari (33)
Abdullah ibn Umar (RA) was known for never promising something without intending to fulfill it. Even in old age, he would walk great distances just to fulfill old promises made in youth.
Fulfilling promises builds trust, honors character, and brings barakah (blessing) in one's affairs.