Sayyiduna Abu Hurayrah (may Allah be pleased with him) met Sayyiduna al-Hasan bin Ali (may Allah be pleased with them both) and said: "Raise your garment so that I may kiss where I saw the Prophet (peace and blessings be upon him) kissing." So he lifted it from his belly, and he (Abu Hurayrah) placed his hand on his navel.
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2751