Musnad Ahmad 234:1

'Abd al-Samad ibn 'Abd al-Warith narrated to us, he said: My father narrated to me, he said: Layth narrated to us, from Talhah, from his father, from his grandfather, that he "saw the Messenger of Allah, may Allah bless him and grant him peace, wiping his head until he reached the *qadhal* (occiput), and what follows it from the front of the neck, once." He said: The *qadhal* is the nape of the neck.