Reb Tzadok of Kopust was a chossid of the Mitteler Rebbe and Tzemach Tzedek who was renowned for his avodas hatefilah.
Too young to have been a member of the Alter Rebbe's chadorim, R. Tzadok still managed to learn what the Alter Rebbe expected of Chabad chassidim. They would call him "Tzadok the davener," due to his davening at great length.
R. Tzadok chose to earn a living through managing a windmill, not a watermill. Since windmills were only active several months a year, he could dedicate the rest of the year to Torah and avoda. His mill was not far from Lubavitch, and whenever his mill was not active, he would spend time in Lubavitch.
As a youngster, the Rebbe Maharash asked R. Tzadok why he davened so long. R. Tzadok replied, "I am fulfilling the Torah's instruction to 'retell the going out of Mitzrayim,' since during davening one goes out of his limitations (meitzarim)."
The Tzemach Tzedek called him "Tzadok the Baal Tefilah," since he was truly a master of davening, which came from his allowing the davening to become his master.
(סה"ש ת"ש-תש"א ע' 247)
Whenever he traveled through Kopust, R. Hillel Paritcher would stay at R. Tzadok's home. R. Hillel would say that R. Tzadok is a chossid according to the wishes of the Alter Rebbe, the Mitteler Rebbe and the Tzemach Tzedek.
(בית רבי ע' 337)
