Riyad as-Salihin

Ibn Mas'ud, Ibn 'Umar and Anas (May Allah be pleased with them) said: The Prophet (ﷺ) said, "For every one who breaks his covenant, there will be a (huge) flag on the Day of Resurrection and it will be said: 'This flag proclaims a breach of covenant by so-and-so."'

[Al-Bukhari and Muslim].