Abu Ammar (may Allah have mercy on him) narrated from Sayyidah Umm al-Fadl (may Allah be pleased with her) that she came to the Prophet (peace and blessings be upon him) and said: "O Messenger of Allah, I have seen a disturbing dream." He (peace and blessings be upon him) said: "What is it?" She said: "May Allah keep you well, it is grave." He (peace and blessings be upon him) said: "What is it?" She said: "I saw as if a piece of your body was cut off and then placed in my lap." The Messenger of Allah (peace and blessings be upon him) said: "You have seen a good dream. Fatimah (may Allah be pleased with her) will, if Allah wills, give birth to a son who will be brought up in your lap." So Fatimah (may Allah be pleased with her) gave birth to Hasan, and he was brought up in her lap. I took him to the Prophet (peace and blessings be upon him) and set him down, and he urinated on him (peace and blessings be upon him). I went to pick him up, but he (peace and blessings be upon him) said: "Leave my son, for my son is not impure." Then he (peace and blessings be upon him) called for water and poured it over it.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21505