Musnad Ahmad 32:807

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Hisham ibn Zayd, who said: I heard Anas ibn Malik narrating: That when his mother gave birth, they took the boy to the Prophet, may Allah bless him and grant him peace, so that he might perform the *tahnik* (chewing a date and rubbing it on the palate of the newborn) for him. He said: "And the Prophet, may Allah bless him and grant him peace, was in a sheep pen, branding the sheep." Shu'bah said: "And the extent of my knowledge is that he said: 'on their ears'."