עשר תעשר את כל תבואת זרעך
“You shall separate a tenth from all the crop of your land” (Devorim 14:22)
The Torah commands us to tithe our crops. The Gemara in Taanis (9a) interprets this pasuk by playing on the similarity between the letters “shin” and “sin.” It renders the words עשר תעשר in our verse as עשר בשביל שתתעשר - tithe and you will become rich. What source is there for the Gemara’s teaching that tithing will make a person wealthy?
The Vilna Gaon notes that the Gemara (Bava Metzia 31a) generally understands a doubled verb as requiring a person to repeatedly do the action referred to as many as 100 times. In other words, he is not absolved from his obligation by performing it once. He must repeatedly do the mitzvah as many times as is necessary. In this light, this pasuk, with its double command to tithe (עשר תעשר), should be understood as requiring a person to tithe his money as many as 100 times.
The Gemara in Kesuvos (50a) records that Chazal decreed that a person should not give more than one-fifth of his money to tzedokah. If so, the Gemara in Taanis questioned how a person could be permitted to tithe by giving one-tenth of his money even three times, as this would require him to give more than one-fifth of his assets to tzedokah. To resolve this concern, the Gemora answered that the Torah guarantees that a person who does so will become rich and will have enough money to continue tithing – even 100 times – without ever falling below the threshold of having given one-fifth of his original possessions to tzedokah. (R’ Ozer Alport)
