Musnad Ahmad 28:551

Ya'qub narrated to us, my father narrated to us, from his father, from Humayd ibn 'Abd al-Rahman, I heard 'Abdullah ibn 'Amr saying: The Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, among the greatest of the major sins is that a man curses his parents." They said: "O Messenger of Allah, and how does a man curse his parents?" He said: "A man insults another man, so he insults his father, and he insults his mother, so he insults his mother."