Yes I remember reading that from HH Pabongkha Rinpoche's biography online here, let me copy it here so that everyone can have a read about the incident. Thank you Wisdombeing for a great reminder about the incomparable HH Pabongkha Rinpoche
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I personally think it was the 13th Dalai Lama's way to show the greatness of Pabongkha Rinpoche to others by "challenging" Pabongkha Rinpoche in that way. You should read the one about HH the 13th Dalai Lama asking Pabongkha Rinpoche to make rain. I said this because the 13th Dalai Lama requested Kyabje Pabongka to give the yearly Lamrim teachings in 1925 (usually it's the Gaden Tripa who teaches and the teachings is for 7 days), and that year, it went on for 11 days
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From His Holiness Pabongka Rinpoche’s Biography on dorjeshugden.comhttp://dorjeshugden.com/wp/?p=3965There was one incident in which Pabongka was summoned by His Holiness to prove that his Southern Lamrim Tradition called, Shargyu, was genuine. Somehow this tradition was not popular in Tibet at that time, and very little was known about it. Therefore, one could not find much proof of its source. It was not written anywhere in the regular texts studied in the monasteries. There were even talks amongst learned scholars of Pabongka introducing a strange system of a Southern tradition that he learnt from an old monk in some village in a corner of Tibet. Hence, people were trying to refute the authenticity of this Southern style Lamrim. In this respect, His Holiness had to do something to intervene and asked Pabongka to provide solid evidence.
The Southern Lamrim tradition was what Pabongka has studied under Dagpo Rinpoche. Being fiercely devoted to his Root Guru, Pabongka would not tolerate anyone disparaging his Guru’s name or even remotely implying that his Guru was wrong in any way. So, Pabongka had to send a letter in reply to the Thirteenth Dalai Lama’s questions.
At first, Pabongka did not think it was so serious. However, upon hearing that without concrete proof, he would have to declare that his newly introduced Southern Lamrim tradition to be a fraud and no one would be allowed to practice it, he was unwilling to risk any injury to his Guru’s name. So, Pabongka thought for a while and then said to the Manager of his labrang, “Under these circumstances, I shall reply. I shall dictate and you go ahead and take notes”.
Then Pabongka quoted,
“The Buddha said in this sutra and that sutra and in the collected works of the Buddha in volume such and such this is written and right at this moment Your Holiness is sitting in your room and if you look at your back in the third shelf, open that book and read on page 146 at the back side the 6th line, there it says this, this and then, if you look at your left side, on the second shelf, the second volume number, this and this, pull that book out and there it says this, this, this….”
Pabongka continued, “This is the proof from the Kanjur and if you read this book by Asanga
which is available in Your Holiness’s room on such and such a shelf in the outer volume whose colour is this, and the inner book is this, and then, if you look at line this, line that, page number this and this, you will find it. And from the Tibetan tradition, look in the works of your late Master, Purchog Jampa Rinpoche,
in volume number four of his collected works, which is in Your Holiness’s bedroom on such and such a shelf and the colour of the cloth is this and the page number is that.”
Pabongka Rinpoche was so precise and confident in pointing out the details for His Holiness to find every piece of evidence to support his Southern Lamrim tradition. This letter was then given to His Holiness’s Chamberlain to be delivered into the hands of His Holiness.
When the Thirteenth Dalai Lama read it, he asked his Chamberlain to take out the exact books and volumes as described by Pabongka in the letter. Everything was proved right. Then His Holiness asked his Chamberlain, “Did you know and tell him I would be in this room?” The Chamberlain answered, “No.” His Holiness did not comment any further. 