Introduction

Some hold that this law is a biblical obligation, while others hold it is of rabbinic origin.
There are several sources and reasons given for this law 1 :

Sidra Lech Lekha

In Sidra Lech Lekha (Genesis 12:1), the two words, consisting of the same letters in each word; Lech Lekha (Go for yourself) from your land… make a total of 100 = 30 + 20 + 30 + 20. Abraham was commanded by G-d to leave his abode and move over to a place that Hahsem would show him later on - the land of Canaan.
G-d promised Abraham that he would be blessed and also that he himself would become a blessing. Abraham the Hebrew, or Avraham Ha’Ivri in Hebrew, means, , a Abraham who moved over from one place and paradigm of idolatry to another place an paradigm of a single G-d worship.

King David

King David re-established this custom and Halakha to recite 100 blessings per day to stop the plague that was killing a 100 people every day. This Halakha was transmitted to Moses at Sinai but got forgotten over the many years that passed by.

Tzadik (A righteous person)

The Gematria (numerical value) of each 4 letter word Tza-D-I-K = 90 + 4 + 10 + 100, which indicate the following: 90 times per day we answer Amen to blessings, 4 times per day we recite Kedusha (3 in the morning and one in Mincha), 10 per day we answer to Kadish – Amen, Yehe Shmei Rabba + 100 blessings we should recite daily 2 .

100 corresponds to one hundred sockets of the Mishkan (Tabernacle) 3

Other sources

The Babylonian Talmud (Gemara Menachot 43b) learns about 100 blessings from the verse Devarim 10:12); "And now, O Israel, what (Heb. Mah) does the LORD your God demand of you? ..." Says the Gemara; Do not read Mah but read Meaah (100), as the complete verse contains 100 letters. Also, Mah in Gematria using At-Bash (10 + 90) is 100 4 .

The verse "Ki Khen Yivorach Gaver who is G-d fearing" (Tehillim 128:4), the first two words, "Ki Khen" make a total numerical value of 30 = 70 + 100.

The 100 blessings per day are a fence against the 100 afflictions mentioned in Sidra Ki Tavo; 98 curses PLUS two other illnesses mentioned by the words; Kol Choli (any illness) and Kol Macca (any blow/smite) - Deuteronomy 28:61, which are not included in the 98 curses 5 .

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