R. Reuven Hasofer of Yanovitch was the Alter Rebbe’s personal sofer. The Alter Rebbe taught R. Reuven how to write the letters and also how to manufacture the ink, and he would only use parshiyos written by R. Reuven. The parshiyos written by R. Reuven are the primary source for the Alter Rebbe’s ksav.
Someone once tried to slander R. Reuven to the Alter Rebbe by saying that he mixes some ingredient into his ink to make it shine. The Alter Rebbe replied, "Indeed, I know about that. He mixes yiras Shomayim in his ink, nothing else!"
R. Aizil Homiler related:
The Alter Rebbe once told his son the Mitteler Rebbe, "I am going to lie down. Stand by the door and wake me up for Mincha." Standing by the door, the Mitteler Rebbe was surprised to see the bed moving about.
The Alter Rebbe suddenly walked out of the room and said, "Quick! Call Reuven the sofer to come with a retzuah." When R. Reuven came, the Alter Rebbe showed him his new method for tying the knot on the head tefillin.
When R. Yehoshua Falk Gurary (older brother of the Rashag) was in Lubavitch, someone offered to sell him parshiyos written by R. Reuven for a high price. R. Yehoshua replied that he would like to show them to the Rebbe Rashab, and if the Rebbe confirms that they are R. Reuven's writing, he will consider it.
R. Yehoshua brought them to the Rebbe Rashab who confirmed that they were unquestionably R. Reuven's writing and added, "Take them, you need them! If you don't buy them, I will." Hearing this from the Rebbe, he gladly paid the price.
The Alter Rebbe once sent a messenger to bring a mezuzah from R. Reuven. The messenger came to R. Reuven and asked for two mezuzos: one for the Rebbe and one for himself.
When the sofer gave him the mezuzos, he told him which one to give to Alter Rebbe. The messenger figured that the Rebbe's mezuzah was better, so he switched them and took the Rebbe's mezuzah for himself.
When the Alter Rebbe received the other mezuzah, he looked at it and expressed surprise that this is what the sofer had sent him. The shliach was ashamed and told the Rebbe that indeed the sofer had sent another mezuzah. The Rebbe took one look at that one and said, "This is a mezuzah!"
Seeing the messenger's disappointment with his inferior mezuzah, the Alter Rebbe reassured him that they were identical aside from one difference: at the Alter Rebbe's request, his mezuzah was not written on a cloudy day.
