Musnad Ahmad 28:526

'Affan narrated to us, Hammad ibn Salamah narrated to us, Sa'd ibn Ibrahim narrated to us, from Humayd ibn 'Abd al-Rahman ibn 'Awf, from 'Abdullah ibn 'Amr, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, the greatest of the major sins is the disobedience of parents." He said: It was said: "And what is the disobedience of parents?" He said: "A man insults another man, so he insults his father, and he insults his mother, so he insults his mother."