Musannaf Ibn Abi Shayba 17:1274

Ibn ‘Uyaynah narrated to us, from Ibn Abi Najih, from his father, reaching the Prophet (may Allah bless him and grant him peace), who said: "Indeed, the most grievous of usury is a man’s encroachment upon the honor of his brother through verbal abuse. And indeed, the greatest of the major sins is a man cursing his parents." It was said, "O Messenger of Allah, and how does he curse his parents?" He said, "He insults people, so people insult his parents in return."