As per the historical tradition, Guru Angad Sahib started the tradition of singing Asa Ki Var in the early mornings in the court of Guru Nanak Sahib. The original source of information on the revelation of Asa Ki Var is janamsakhi literature. According to story number thirty-two of Puratan Janamsakhi (writing time - 1657-58; writer -Bhai Saido; place of writing – ‘Cara? Majhar Ke’ near Peshawar), Asa Ki Var was uttered in discourse with Sheikh Braham. Sheikh Braham, or Ibrahim, was the twelfth successor of the revered Sheikh Pharid who reigned from 1511 to 1552 CE.

The story in the Puratan Janamsakhi goes like this:

“...[the Guru] walked through the wilderness along the river Ravi and reached the region of Pattan [in Pakistan]. The Guru camped in a wilderness about six miles from Pattan; Bhai Mardana was along. There was a seat of a Sufi master Sheikh Farid in Pattan, on which his descendant Sheikh Braham was enthroned. One morning, Sheikh Kamal, a disciple of Sheikh Braham, came to the jungle to collect firewood. He noticed that the Guru and Bhai Mardana were sitting there. Then Bhai Mardana started playing the rabab and uttered a salok in Rag Asa.

“salok
(O Divine!) You Yourself are the slate, Yourself the pen and Yourself the writing on it also.
O Nanak! Only the One Divine should be praised; why praise the other (who takes birth and dies)? -Guru Granth Sahib 1291

“When Sheikh Kamal heard the salok, he kept the wood aside and approached them. He requested the Guru to direct Bhai Mardana to repeat the salok. Bhai Mardana sang the salok again. Kamal learnt the salok by heart, bid farewell to them and left with whatever firewood he had collected. After reaching Pattan, he kept the wood aside and went to his master Sheikh Braham. Kamal bowed to Sheikh Braham and told him that he had met a lover of Khuda. Then Sheikh Braham asked him, “Where did you meet him?” Kamal replied, “Master! I had gone to collect wood. His name is Nanak and he is accompanied by a rabab player. He utters his own saloks. Then Sheikh Braham asked, “Child! Did you learn any salok?” Kamal replied, “Yes, I have learnt one!” Sheikh Braham said, “Recite, let me see how is it?” Then, Kamal replied that he says:

“(O Divine!) You Yourself are the slate, Yourself the pen and Yourself the writing on it also.
O Nanak! Only the One Divine should be praised; why praise the other (who takes birth and dies)? -Guru Granth Sahib 1291

“Hearing the salok Sheikh Braham asked Kamal, “Did you understand anything?” Kamal replied, “You know everything, O Master!” To which Sheikh said, “Child, the man who has uttered this salok appears to be a Divinely person. Take me to him; he has praised Khuda!”

“Then Sheikh Braham took Kamal along and went to Guru Nanak. Sheikh Braham reached where the Guru was sitting and greeted him. The Guru also greeted him and said, “Divine has been benevolent, who has arranged our meeting.”

“Then Sheikh Braham said, “I am wonderstruck by one of your salok. I said, take me to the one who has uttered this salok.” The Guru then replied, “I am blessed to have met you!” Then Sheikh Braham said, “Please explain your salok in which you state that there is only One, why praise the other?”

“But, there is one Master and two instructors.
Which one to serve/follow and which one to discard?

“You state that there is only One; but Hindus say that theirs is true and the Muslims say that theirs is true. Please explain, let me see, who do you consider to be right and who do you consider to be wrong?” Guru Nanak replied, “There is only one Master and only one instruction; serve only the One and reject the other.”

“… Then Sheikh Braham asked, “Please recite a ballad (var) of Khuda for us. Our understanding is that a ballad cannot be without two [characters] and you talk about one and only One. Let me see, who do you take as a rival/parallel of Khuda!” Then, Guru Nanak directed Bhai Mardana to play the rabab. Bhai Mardana played the rabab in Rag Asa. The Guru uttered salok:

“I devote (myself) to my Guru… witnessed Your Own play delightfully.
“Nine pauris were uttered regarding this...

The witnessed-narrative (sakhi) number sixteen in the Janamsakhi B-40 also contains this narrative. According to Dr. Kirpal Singh, Guru Sahib met Sheikh Ibrahim twice; once while going towards the east, and the second time while going towards the west. Asa Ki Var was revealed during their second meeting. To support their claim, they have quoted Bhai Mani Singh’s Janamsakhi. There is a reference to the meeting between Guru Nanak Sahib and Sheikh Ibrahim during the former’s third travel (udasi) in the Meharban Janamsakhi as well. In this Janamsakhi, paramarth (essence) of twelve pauris is given in the form of discourses. Prof. Sahib Singh also agrees that the Guru’s visit occurred during his third udasi.

According to the Puratan Janamsakhi, the pauris that came after pauri nine were revealed during the meeting with Duni Chand. Duni Chand was a resident of Lahore. He had prepared a braham bhoj (religious feast) to satisfy the hunger of his ancestors. The Guru went to him and counselled him. Thus, Janamsakhi literature points to the revelation of the pauris of Asa Ki Var occurring in Pakpattan and Lahore. But, while quoting Gian Ratnavali of Bhai Mani Singh, Bhai Manjit Singh states that Asa Ki Var had been revealed even before meeting Sheikh Braham (Ibrahim):

“Sheikh Braham said: ... The minstrels sing praises of the kings…who are pleased by it. Warriors are also pleased and inspired to fight…”

“The Guru said: Just as they ... sing praises of the king and … the king is pleased and … [similarly] those who are seekers, their mind and senses are also kindled and inspired…”

“Sheikh Braham: ... So, please recite a ballad of the Supreme Being for me…”

“The Guru said: Mardana! Kabir has used Rag Gauri… and for us all rags are [equal], … but this Pir has come with a hope (asa), so… recite a ballad in Rag Asa.
[It is evident from the word ‘recite’ appearing in the above excerpt from Bhai Mani Singh’s manuscript, Gian Ratnavali, page 30-31, that ‘Asa Ki Var’ had been revealed to Guru Nanak Sahib even before meeting Sheikh Braham.]”

As per Prof. Sahib Singh “the subject of the entire var is the same, wherever it was revealed, it was revealed all together.” Other than these citations, there is no other proof available that can shed light on the time and place of this revelation.