It is narrated from Sayyiduna al-Sa'ib bin Yazid (may Allah be pleased with him) that when the son of the Prophet (peace and blessings be upon him), Tahir, passed away, tears began to flow from his (peace and blessings be upon him) eye. It was said: O Messenger of Allah, are you weeping? The Prophet (peace and blessings be upon him) said: "The eye sheds tears, the tears overcome, and the heart grieves, but we do not disobey Allah, the Mighty and Majestic."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6637