As I Heard Them, Vol. 1, pp. 452-453, no. 54
Continued from the biography of R. Zalman Duchman:
At the end of the Simchas Torah farbrengen of 5726 (1965), when it was time to begin hakafos, the Rebbe instructed R. Zalman to announce " Sha, shtiller!" and get the crowd quiet. After several attempts, the Rebbe asked him to announce three times, "Kol mevaser, mevaser v'omer!" ("a voice announces and says" from the Hoshanos liturgy). As he did, the Rebbe smiled.
When the Rebbe requested at one Simchas Torah farbrengen for the children to sing "Hamal'ach Hagoel," R. Zalman led the singing, to the Rebbe's great delight.