It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that Sayyiduna Husayn (may Allah be pleased with him) was brought to the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) took him in his lap and began to kiss him. Meanwhile he urinated, and the people moved to lift him away, but he (peace and blessings be upon him) said: "Do not interrupt his flow." So he (peace and blessings be upon him) left him until he had finished urinating.
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7668