Tehillim & Chassidim
Excerpt from Law and Custom in Hasidism:
The entire volume of Tehillim was never recognized as a single unit, the reciting of which should be obligatory or a segulah (i.e. an efficacious means toward obtaining a desired goal). Only in the latter part of the Middle Ages do we find the first statements in the name of R’ Avigdor of Regensburg and Rabbeinu Ephraim which speak of the Book of Tehillim and of the segulah of reciting it as a single unit.
From the various sources scattered here and there on the particular segulah of the book as a way to protect oneself from the destructive angels, it appears that its importance grew greatly among the people under the influence of the kabbalists. It was especially valued by the later kabbalists of the Ari’s school. They instituted the recitation of Tehillim on the night of Hoshana Rabbah, and composed a Yehi ratzon prayer to be said throughout the year after concluding the recitation of each of the five books into which Tehillim is divided. For Hoshana Rabbah, there are special versions to be recited after the conclusion of each book. Through the kabbalists, the Book of Tehillim became beloved by the people as a whole, and whenever there were troubles or a calamity which befell the community or an individual, they would pour out the bitterness of their heart through this book. In it, they found consolation, strength, faith and hope.
It is therefore not suprising with the growth of Hasidism, which followed the footsteps of Kabbalah, the value of this book increased, and the Hasidim considered the reading of Tehillim to be of great importance. But one should not seek the source of this only in the influence of Kabbalah on Hasidism, for this minhag is primarily a Hasidic product. After all, Hasidim attempted to lift the spirit of the simple Jew and to implant within him the recognition that he was equal to the “lamdan” in value and status, and that every Jew can be as righteous, or even more righteous, than the lamdan, as long as whatever he does is for the sake of Heaven. There is nothing better for this than the Book of Tehillim, for our Sages tell us, David asked that those who recite it should receive the same reward as those who study Nega'im and Ohalos. If so, the simple Jew who recites Tehillim is no less worthy than the Jew who studies the most difficult passages of Talmud.
There were tzaddikim who recited all of Tehillim over the course of each week, while others completed it twice a week, once over the course of the week and once over the course of Shabbos. They especially made a point of reciting Tehillim during the month of Elul and the Ten Days of Repentance, on the night of Yom Kippur and the night of Hoshana Rabbah, According to the tradition of the Hasidim, the Baal Shem Tov instituted a special group of Tehillim to be recited Erev Rosh Hashana at the graves of tzaddikim, these being: 4, 7, 11, 12, 13, 22, 23, 24, 38, 39, 40, 42, 43, 51, 86, 90, 91, 102, 103, 141, and 142, and thereafter one recites from Tehillim from 119 (whose verses begin with the letters of the Hebrew alphabet, eight verses for each letter) those verses which spell out one's name (for example, if one's name is Avraham he should recite the verses that begin with the letters alef, beis, reish, heh, mem) and then the verses which spell out Shadai K'ra Satan.
The great Hasidic masters also used the reciting of Tehillim as repentance for various sins. There are many stories in the Hasidic literature about this, showing the power of the Book of Tehillim in the forgiveness of sins. Not only in the the aggadic literature of the movement, but in the halachic words of Hasidim as well, one finds this - as actual halacha. A famous tzaddik answered a question about "a mohel, where of the children whom he had circumcised had died, and who is afraid that he was negligent in that he circumcised the child while he was jaundiced" and did not wait "to postpone the circumcision until his blood had settled down." The mohel now asked what he could do as penance for having killed the child through negligence. The tzaddik answered [She'elos U'Teshuvos Cheishiv Moshe, Yoreh Deah 49] that he was to fast forty days over a period of time, and that on each day that he fasted was to recite all of Tehillim slowly, from the beginning to the end. Furthermore on the yahrzeit of the infant each year "he is to say all of Tehillim" and study Mishnah in memory of the infant, for the infant, too, might need a tikkun from a previous gilgul (incarnation). If he accepts this, his sin will be forgiven, and from that day on he is to examine each infant on the day prior to the scheduled cirmcumcision.