The Tzemach Tzedek told his son, my grandfather, that the maamar Umareihem uma'asseihem in Torah Or, Yitro, is the first discourse the Maggid delivered when he assumed leadership of the chassidim, on Shavuot 5521 (1761). The Alter Rebbe heard the maamar from R. Mendel Horodoker,1 author of Pri Haaretz, who had been present that Shavuot in 5521. However, the Alter Rebbe expounded the maamar in his own style.