The famous Lubavitcher gaon, Rabbi Shneur Zalman Fradkin, commonly referred to as the “Toras Chessed” after his famous work, was a close friend of Reb Tzadok of Lublin, who served alongside him as Rabbi of Lublin.
Reb Tzadok would often visit Rabbi Shneur Zalman to discuss Torah concepts and the proper manner in which to serve G-d.
Once, Rabbi Shneur Zalman invited Reb Tzadok to his home Friday afternoon after the Mincha prayers.
When Reb Tzadok arrived, Rabbi Shneur Zalman placed his hands on his shoulders. The two began pacing the room discussing deep secrets of the Torah and remained that way for five consecutive hours!
(HaGaon MiLublin, Pg. 38)
