Sheikh al-Sadouq (ra) narrates that Saad Ibn Abdullah al-Qommi asked a bunch of questions to Imam al-Mahdi (atfs) and among then was the meaning behind the
first verse of Surat Maryam: "Kaaf, Ha, Ya, 'Ayn, Sad." - Quran (19:1)
The Imam (atfs) said: 'Prophet Zakraiya (as) prayed for Allah (swt) to teach him about the five names. Then archangel Gabriel descended to him and taught him the names. So whenever Prophet Zakraiya mentioned the names of Prophet Muhammad, Sayeda Fatima, Imam Ali and Imam Hassan, his concerns would fade and his anguish would be relieved. But whenever he mentioned the name of Imam Hussain, he would get sad.
So he (Zakraiya) said: O' my Lord! Why is it when I mention the names of the four I get relieved from my concerns, yet when I mention the name of Hussain, my eyes tear up?!
So Allah (swt) revealed Kaaf, Ha, Ya, 'Ayn, Sad. As for Kaaf, it stands for Karbala. As for Ha, it stands for the tragedy of Hussain (halak al-Hussain).
As for Ya, it stands for Yazid who will oppress Hussain. As for 'Ayn, it stands for the thirst of Hussain ('atash al-Hussain). And Sad, it stands for the patience of Hussain (Sabr al-Hussain). Upon hearing this,
Prophet Zakraiya wouldn't leave his mosque for three days, and he spent those days weeping and wailing.
Then Zakraiya said: "O' my Lord! Grant me a child that will be the light of my eyes when he grows up, and make him my inheritor and my successor. And make him at the same level as Hussain." So then Allah (swt) gave Zakriaya his son Yahya, and Yahya's mother pregnancy lasted 6 months just like Fatima's pregnancy with Hussain.
(Source: Kamal al-Deen by Sheikh al-Sadouq, page 461).