Abu Hazim says: I was present with Sayyiduna Imam Husayn (may Allah be pleased with him) when Sayyiduna Imam Hasan (may Allah be pleased with him) passed away. He (that is, Sayyiduna Imam Husayn (may Allah be pleased with him)) was taking hold of the neck of Sa'id bin al-As and pushing him forward, saying: "Go forward; had this not been the Sunnah, I would not have put you forward." (The narrator says:) In those days Sa'id bin al-As was the governor of Madinah.
Al-Tabarani recorded it in al-Mu'jam al-Kabir, and al-Bazzar also recorded it, and its narrators have been declared trustworthy.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الجنائز / 4146