Musannaf Ibn Abi Shayba 17:1273

Ghundar narrated to us, from Shu'bah, from Sa'd ibn Ibrahim, from Humayd ibn 'Abd al-Rahman, from 'Abdullah ibn 'Amr, from the Prophet (peace be upon him), who said: "Indeed, the greatest of sins in the sight of Allah is that a man curses his parents." They said, "And how does a man curse his parents?" He said, "He curses a man's father, so he curses his father, and he curses his mother, so he curses his mother."