Musnad Ahmad 30:3196

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