Kumayl Ibn Ziyad Nakha'i was a confidant amongst the companions of Amir al Muminin, Imam Ali Ibn Abi Talib (A.S.) and this sublime Du'a was first heard from the beautiful, though anguished, voice of Imam Ali. According to Allama Majlisi (on whom be Allah's Mercy) Kumayl had attended an assembly in the Mosque at Basra which was addressed by Imam Ali in the course of which the night of the 15th of Shaban was mentioned. Imam Ali said-"Whosoever keeps awake in devoutness on this night and recites the Du'a of Prophet Khizr, undoubtedly that person's supplication will be responded to and granted. When the assembly at the Mosque had dispersed, Kumayl called at the house where Imam Ali was staying, and requested him to acquaint him with Prophet Khizr's "Du'a". Imam Ali asked Kumayl to sit down, record and memorise the "Du'a" which Imam Ali dictated to Kumayl.
Imam Ali then advised Kumayl to recite this "Du'a" on every Shab e Juma'h,(i.e. evening preceding every Friday,) or once a month or at least once in every year so that, added Imam Ali, "Allah may protect thee from the evils of the enemies and the plots contrived by impostors. O' Kumayl! in consideration of thy companionship and understanding, I grant thee this honour of entrusting this "Du'a" to thee."
Sheikh Tusi (460 AH / 1067 CE) and Sayyid ibn Tawus (664 AH / 1271 CE) both record this prayer under the acts of worship for the 15th of Sh'ban.
It is highly recommended that after reciting this dua, one must seek closeness to Imam al-Mehdi (may Allah hasten his return) the Hujjah of our Time; Wali al-'Amr; the Greatest Secret (of Allah); the possessor of the divine Occultation and inviter to the message of the Prophet; the Successor of all Prophets and the Successor of all Successors of the Prophets; the physical manifestation of the Justice of Allah; the Flag of Guidance; the one who will destroy all corruption and oppression and those who oppose the truth; the destroyer of the enemies of Allah - the misguided and the disbelievers; the one who will purify the Earth of deviation and worship of the self; the greatest sign of Allah; the one who acts as the connection between the heavens and the earth; our Imam, Master and Qaim, al-Mahdi - may my soul and the souls of all of those in the universe be sacrificed for him.