Imam Ali [a] was sitting with a group of his companions. One of them asked him about the explanation of the following Holy Verse:
Therein every wise affair is made distinct.
The Imam said:
It's on 15th Shaaban.
Then he added:
By Allah, man faces his fate on Shaaban 15th. So, man must implore Allah at such a night. He must recite al-Khidr's supplication. Certainly Allah accepts it.
The meeting was over. The Imam went home.
The evening came. Then it got dark. People went to bed. At that hour, Kumayl got up. He went to Imam Ali's house. He had question.
Kumayl knocked the door. Imam Ali [a] said:
Kumayl, what has brought you?
Kumayl, politely said:
Amirul Mu'mineen, al-Khidr's supplication.
The Imam said kindly:
Kumayl, please sit down.
The he added:
I'll read you a supplication. Read it on Thursday nights.
The Imam began reading the supplication. Kumayl was writing it. Nowadays, millions of Muslims read the supplication. They call it Kumayl's supplication.