R. Mordechai Dov Ber Slonim was the son of Rebbetzin Menucha Rochel, the daughter of the Mitteler Rebbe, and R. Yaakov Slonim. Born in 5600, he was a child when his family moved from Lubavitch to Eretz Yisrael.
A resident of Chevron, R. Mordechai Dov Ber served as one of the directors of Colel Chabad and was a leading figure in the local Chabad community. He was said to closely resemble his grandfather, the Mitteler Rebbe, in his physical appearance. He visited Lubavitch multiple times and developed a close relationship with Rebbe Maharash and Rebbe Rashab. R. Mordechai Dov Ber passed away on 5 Teves, 5676.
Having lived for decades in the Holy Land, R. Mordechai Dov Ber decided to make the trip to Lubavitch to visit the Rebbe Maharash for the first time. Though he and the Rebbe Maharash had played together as children, his devoted hiskashrus pushed him to travel to his Rebbe.
The trip had been a sudden decision, and he did not inform the Rebbe Maharash in advance that he would be coming. After a long journey, he arrived in Lubavitch at 2 am. Certain that the Rebbe was awake, he went straight to the Rebbe's home and knocked on the door. The door opened and there stood the Rebbe who greeted him warmly, "Ah! Shalom aleichem R. Moteh Ber!"
R. Mordechai Dov Ber was understandably shocked that the Rebbe recognized him, having last seen him as a young boy almost forty years ago. The Rebbe addressed his wonderment, "Who else would come and bang on my door at 2 am if not R. Moteh Ber!"
(מגדל עז ע' ריז)