It is narrated in Al-Kisaa Hadeeth that when Imam Hassan (PBUH) entered into the Kisaa (small tent), he…

Answer

In the Name of Allah, be advised, my son, that the holy text is Al-Hassan had already entered before Al-Hussein and the prophet was alone under the Kisaa. Then, the one who enters to a group of people is allowed to greet the best one of them. Therefore, however he greets, his greeting is enough. Allah knows the best.