Allah said about Ismail; انه كان صادق الوعد. This means that Ismail (as) was truthful to his promise.
Awf ibn Uthman said that it’s better for a person to die of thirst than to break a promise. Meaning, if you said you would do something for someone and didn’t do it then you should fulfil your promise.
Imam Ahmad (rh) was asked, how do you know the liars? He said, by their appointments.