Hazrat Sa‘eed bin Abi Sa‘eed Al-Maqburi (rahimahullah) narrates the following incident:
On one occasion, we were present with Hazrat Abu Hurairah (radhiyallahu ‘anhu) when Hazrat Hasan bin ‘Ali (radhiyallahu ‘anhu) came to us and greeted us with salaam. When the group heard him make salaam, they replied, after which Hazrat Hasan (radhiyallahu ‘anhu) departed. However, Hazrat Abu Hurairah (radhiyallahu ‘anhu) was engaged in some work at that time due to which he was unaware that Hazrat Hasan (radhiyallahu ‘anhu) had come and made salaam.
When someone brought it to his attention that Hazrat Hasan (radhiyallahu ‘anhu) had come to the gathering, made salaam and thereafter left, he felt remorseful that he was unaware of Hazrat Hasan’s (radhiyallahu ‘anhu) presence and did not reply to his salaam. He immediately left the gathering and went to look for Hazrat Hasan (radhiyallahu ‘anhu) in order to reply to his salaam. When he found Hazrat Hasan (radhiyallahu ‘anhu), he replied to his salaam saying, “Wa ‘alaika ya Sayyidi (and may salaam be upon you, O my leader).”
A person later on asked Hazrat Abu Hurairah (radhiyallahu ‘anhu), “Why did you address Hazrat Hasan (radhiyallahu ‘anhu) with the title ‘Sayyidi’ (my leader)?” Hazrat Abu Hurairah (radhiyallahu ‘anhu) replied, “The reason for me addressing him with this title is that I was once present with Rasulullah (sallallahu ‘alaihi wasallam) when I heard him referring to Hazrat Hasan (radhiyallahu ‘anhu) saying that he is the Sayyid (leader) of this ummah.”
(Majma’uz Zawaa’id #15049 and Mustadrak Haakim #4792)