Musnad Ahmad 30:344

Muhammad ibn Abi ‘Adi narrated to us, from Ibn ‘Awn, from ‘Umayr ibn Ishaq, who said: I was with al-Hasan ibn ‘Ali, and Abu Hurayrah met us. He said: "Show me, so that I may kiss from you where I saw the Messenger of Allah, may Allah bless him and grant him peace, kiss." He said: So he [al-Hasan] opened his shirt. He said: "And he kissed his navel."