Imam al-Sadiq (a) narrates that the Holy Prophet Muhammad (saw) looked at Imam al-Husayn ibn ‘Ali (pbut) [when he was a child] as he was coming to him and sat him on his lap and said:
“Verily in the hearts of believers there is a heat [of painful mourning] for the murder of al-Husayn [a] that will never extinguish.”
Then Imam al-Sadiq (a) said:
“May my father [a] be sacrificed for him; he is the killed one, who is mourned by every tear (قَتِيلُ كُلِّ عَبْرَةٍ).”
Someone asked him:
“What does ‘the killed one, who is mourned by every tear’ mean, O son of the Prophet of Allah (SWT)?”
He (a) responded:
“No [true] believer remembers him, except he cries [for what befell him]”.
(Al-Mirza al-Nuri, Mustadrak al-Wasail, v.1, pg. 318)