Likutei Moharan, Part II 73:2/ליקוטי מוהר"ן·תנינא ע״ג:ב׳

12. Now, the main refinement of the Twelve Tribes of God—who are the forty-nine letters, corresponding to the forty-nine gates of repentance—occurred in MiTZRayim (Egypt), the concept of MeiTZaR (narrow) of the throat, which corresponds to upper repentance (as brought in the writings of the Ari, of blessed memory; see there and understand well). Therefore, after they were refined in Egypt and merited to depart from there, they counted the forty-nine days of the Omer Counting, which correspond to the forty-nine gates of repentance, the aforementioned forty-nine letters. Then, on the fiftieth day, “God descended on Mount Sinai” (Exodus 19:20). This is the concept of “and I will return to you,” the repentance of God Himself, as it were, which corresponds to the fiftieth gate, as mentioned above.

2And this is: v’eileH shemoT bneI yisraeL haba’iM mitzraymaH eT yaakoV iSh u-beitO (“And these are the names of the sons of Israel who came to Egypt; with Yaakov, each man and his household”), the final letters of which spell TeHiLlIM and TeShUVaH. Through Tehillim one merits teshuvah (repentance), the concept of “the names of the sons of Israel who came to Egypt….” For the forty-nine gates of teshuvah are the forty-nine letters in the names of Israel’s sons. They came to Egypt in order to be refined there.