Musnad Ahmad 28:364

Muhammad ibn Ja'far and Hajjaj narrated to us, they said: Shu'bah narrated to us, from Sa'd ibn Ibrahim, from Humayd. Hajjaj said: I heard Humayd ibn 'Abd al-Rahman, from 'Abdullah ibn 'Amr, from the Prophet, peace and blessings of Allah be upon him, who said: "Indeed, among the greatest of sins 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."