Abu Hazim narrates: I saw Husayn (may Allah be pleased with him), when Hasan (may Allah be pleased with him) died, pushing Sa'id bin al-As forward by the back of his neck and saying: "Go forward; were it not the Sunnah", meaning regard for seniority, "I would not have put you forward." And Sa'id was the governor (amir) of Madinah at that time.
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2899