Musnad Ahmad 28:53

Waki’ narrated to us, Mis’ar and Sufyan narrated to us, from Sa’d ibn Ibrahim, from Humayd ibn ‘Abd al-Rahman ibn ‘Awf, from ‘Abdullah ibn ‘Amr—Sufyan attributed it to the Prophet (marfu’), while Mis’ar stopped it at the Companion (mawquf)—he said: "Among the major sins is that a man insults his parents." They said: "And how does a man insult his parents?" He said: "He insults a man’s father, so he insults his father, and he insults his mother, so he insults his mother."