Abu Sa'id Al-Khudri (May Allah be pleased with him) said

The Prophet (ﷺ) said, "Every one who breaks a covenant will have a flag by his buttocks on the Day of Resurrection. It will be raised higher according to the nature of his breach. Behold, there will be no greater a sin with respect to breaking the covenant than that of a ruler who breaks his covenant with the Muslim masses."

[Muslim].