The Shavuot The Tzemach Tzedek Became Rebbe

When the second Chabad Rebbe, Rabbi Dov Ber (known as the "Mitteler Rebbe") passed away, there were three prospective successors.
Though all three were immensely qualified for the leadership of the Chabad movement, all three unanimously declined all the importuning of the Chasidim. These three were: Reb Chaim Avraham, the brother of Rabbi Dov Ber and youngest son of the Alter Rebbe, Reb Menachem Nachum, the Mitteler Rebbe's son, and the Tzemach Tzedek, the son of the Alter Rebbe's eldest daughter.
As time passed, the pressure among the Chasidim to find a successor escalated, though it seemed that no solution was in sight. Finally, despairing of a solution being found, two of the Chasidim declared, "It is impossible to be without a Rebbe!" They decided to travel to Ruzhin with the intention of accepting the Ruzhiner Rebbe as their Rebbe.
The Ruzhiner Rebbe, Reb Yisrael, was the grandson of the Mezritcher Maggid, and so highly thought of for his enormous piety that he was called the "Holy Ruzhiner."
These two Chasidim travelled to Ruzhin for Shavuot. As was the custom there, (as well as among many other Chasidim) the Ruzhiner distributed shirayim -- food from his table -- to his Chasidim. It was Yom Tov and the Ruzhiner began to distribute wine from his own cup to each of the Chasidim. The two Chabad Chasidim also wanted to participate and receive wine from the Ruzhiner and they proffered their cups for the "cup of blessing."
The Ruzhiner, however, refused them, saying, "If you want some wine, you may take it yourself, but I will not give it to you."
The two were very surprised and protested, "Why won't you give it to us, after all we have come here in order to accept you as our Rebbe?"
Upon hearing those words, the Ruzhiner sat down at the table and began to deliver a deep Chasidic discourse based on the theme, "The Giving of the Torah began, not at Mount Sinai, but at the burning bush."
He explained in great depth that when G-d gave Moshe the task of taking the Children of Israel out of Egypt, G-d told Moshe to "tell the Jews that I have remembered you and want to take you out of Egypt."
Moshe's reaction was strange. He replied that he was afraid the Jews would ask him what is G-d's name. To this G-d replied, "Tell them My Name is, 'I will be what I will be.' "
The Ruzhiner posed the question, "Why did Moshe ask this question of G-d? For Moshe did know G-d's name as he had been handed down a tradition that it was spelled Yud-Kei-Vav-Kei[1]! And why did G-d answer, 'I will be what I will be.'"
The Ruzhiner elucidated the point through the use of numerical equivalents which are often used to explicate texts. He explained that the numerical equivalent of Yud-Kei-Vav-Kei is 26, while that of the words "I will be what I will be" equals 441 which is " emet" -- truth. G-d desired that Moshe be able to reveal to the Jews the truth.
"The word 'emet'," continued the Rebbe, "is also an acronym for, "Torat Menachem Emet" ["the Torah of Menachem is truth"].
When the two Chabad Chasidim heard these words being spoken by the Holy Ruzhiner, they realized that he was intimating that they should return home to the city of Lubavitch and that the Tzemach Tzedek, whose name was Menachem Mendel, should become Rebbe.
Upon arriving in Lubavitch two weeks later, the Tzemach Tzedek had already acquiesced. The returning Chasidim repeated to their fellows the discourse they had heard from the mouth of the Holy Ruzhiner in regard to the word "emet" intimating that the Tzemach Tzedek should be the Rebbe.
The Chasidim recalled with amazement that the Tzemach Tzedek had delivered the same discourse that very same Shavuot, but when he reached the part which identified the acronym of emet with his name, Menachem, he merely hesitated and smiled to himself. Now, they all understood why he had smiled.