Poetry was the main way to register important events at that time. Hassaan bin Thabit made and recited his poetry on the occasion of Ghsdeer stating that the Prophet announced on the Day of Ghadeer at Khum that for whom I am the master, Ali is his master يناديهمُ يوم الغدير نبيهم. بخُمٍّ وأسمِع بالنبي مناديا
فمن كنت مولاه فهذا وليُّهُ. فكونوا لهُ أتباع صِدقٍ مواليا
This poetry has been registered in the history being from an eye witness and it has been mentioned in Sunni and Shia books of history and faith.
It puts big question in front of every Muslim: What did you do after this prophetic announcement? Did you follow Imam Ali as the Prophet ordered? Or you followed others?