Midrash for Exodus 31:13

Raba b. Mechasia in the name of R. Chama b. Guria who spoke in the name of Rab said: "He who bestows a gift on his friend should let him know it; for it is said (Ex. 31, 13.) That ye may know that I am the Lord who doth sanctify you." We have also a Baraitha to the same effect: That ye may know that I am the Lord who doth sanctify you, i.e.. The Holy One, praised be He! said unto Moses "I have a good gift in my treasury; its name is Sabbath; this I wish to bestow on Israel; go, and announce it to them." "We learn from this then," said Rabban Simon b. Gamaliel, "that if one give a piece of bread to a child he ought to make it known to the child's mother." What shall one do [to inform the mother]? Abaye said: "He should put some oil around the child's eye or stain it with some dye" [so that his mother will ask him concerning it, and he will then tell her he also gave him bread]. Nowadays, however, when such is the apprehension of performing an act of witchcraft, what shall one do? R. Papa said: "He should put some of this very thing he gave to the child around his eyes." Is it so? [that a gift should first be told of]; has not R. Chama b. Chanina said: "He who bestows a gift on his friend should not let him know of it, for it is said (Ex. 34, 29.) And Moses did not know that the skin of his face shone, because He had spoken with him." This is not difficult to explain. The latter instance speaks of a thing that may become known by itself, but the former speaks of a thing that cannot become known by itself. Is not the Sabbath a thing that was to be known [and yet God informed Israel of it]? But the reward [for observing it] cannot be known.

R. Chisda held in his hand two priestly gifts from the meat of an ox and said: "I will give these to the man who will tell me one new law in the name of Rab." Whereupon Raba b. Mechasia said to him: "Thus, has said Rab, 'He who bestows a gift on his friend should inform him of it, for it is said (Ex. 31, 13.) That you may know that I am the Lord who doth sanctify you.'" So R. Chisda gave him the gifts. "Art thou so fond of the teachings of Rab?" asked Raba b. Mechasia. "Yea," he responded. Whereupon Raba b. Mechasia said: "This is what Rab said, 'Garments are precious to those who are invested with them, [because they know the value of it].'" "Did Rab indeed say so?" asked R. Chisda. "Surely the last thing is better than the first one, and if I had other gifts I would bestow them on you."

Observe the reward given for the observance of the Torah from the example of Pharaoh. Because Pharaoh said to Joseph, I am Pharaoh (Gen. 41:44), Joseph was exalted by him exceedingly, as it is said: And Pharaoh took off his signet ring and put it upon Joseph’s hand (ibid., v. 42). How much greater, then, is the reward bestowed by the Holy One, blessed be He, who with every commandment declares, I am the Lord (Exod. 31:13). (What else do we learn from this?) We learn from this that there is no limit to the reward given for the observance of the Torah. David foresaw this and declared: Oh how abundant is Thy goodness which Thou has laid up for them that fear Thee (Ps. 31:20).