AL-BAQARAH (The Cow) - 8

 “…and fulfill the covenant with Me, and I shall fulfill your covenant…” (2:40)

This verse tells us that it is obligatory to fulfil the agreement one has entered into, and it is forbidden to break one's promise. The Holy Prophet ﷺ said;

“Those who break their promises would, before being finally punished in the other world, be humiliated before the whole human race when it assembles together on the Day of Judgment, for a flag would be placed as a stigma beside everyone who has committed this sin, and the bigger the crime, the higher would the flag be.”