Matthew 6:3

But when thou dost alms,.... Do it so privately, and with so much secrecy, that, if it was possible, thou mightest not know it thyself, much less make it known to others:

let not thy left hand know what thy right hand doth; acquaint not thy nearest and dearest friend with it; let not one that sits at thy left hand know what thou art doing with thy right hand; it is a proverbial and hyperbolical phrase, expressing the secrecy of the action. It is a Jewish canon p, that

"he that gives a gift to his friend out of love, may make it known, אבל לא בצדקה, "but not if it be by way of alms".''

p Piske Tosephot in Sabbat. c. 1. art. 134.