Musnad Ahmad 30:2383

Isma'il narrated to us, from Ibn 'Awn, from 'Umayr ibn Ishaq, who said: I saw Abu Hurayrah meet Al-Hasan ibn 'Ali. He said to him: "Uncover your stomach, where I saw the Messenger of Allah, may Allah bless him and grant him peace, kissing it." He said: So he uncovered his stomach, and he kissed it.