Midras Imamat 5:1: Sifra & Ein Yaakov (Glick Edition)

1) R. Yossi Haglili says: What is the intent of "and he was a witness or saw or knew, if he does not tell, then he shall bear his sin"? It applies only to testimony that can consist in seeing without knowing or in knowing without seeing, and this obtains only with a monetary claim. ("seeing without knowing": as when one says to another: "I counted out a maneh to you in the presence of so and so, who witnessed this, but I did not apprise them whether it were a gift or a loan or a pledge," and the other says: "Let them come and testify that you counted it out to me in their presence and I will pay you." ("knowing without seeing": as when one says to another: "You admitted owing me a maneh in the presence of so and so," and the other says: "Let them come and testify to this, and I will pay you." R. Akiva says (on Vayikra 5:5: "and he be guilty for one of these"): There are among these that for which he is liable (for withholding testimony) and that for which he is exempt. How so? If money is claimed, he is liable; if anything else, he is exempt.

1) (Vayikra 5:1) ("And if a soul sinned and heard the voice of an oath, and he was a witness or saw or knew — if he does not tell, then he shall bear his sin.") If it were written (only) "he heard the voice," and not "of an oath," I might think the intent of the verse to be that if one overheard a man telling his neighbor "Let us go and serve idolatry," and he could have testified in beth-din to this effect and failed to do so, ("I might think") that he is liable. — Now if one who (only) says this ("Let us go and serve, etc.") is not liable, should the one who just overhears it be liable (for withholding testimony)! It must be, then, that the verse is speaking of one being besworn in beth-din not to withhold testimony, and withholding it, in which instance the sayer (i.e., the beswearer himself) is not liable (even if he knows that the oath will be violated) and the hearer is liable (for its violation).

2) You, too, do not wonder that even though one who tells his neighbor "Let us go and serve idolatry" is not liable, the hearer (if he does not testify to this) is liable. To this end, it is written "and he heard the voice of an alah," an alah being an oath, viz. (Bamidbar 5:21): "Then the Cohein shall beswear the woman with the oath of the alah." This tells me only of an oath accompanied by an alah (lit., a curse). Whence do I derive the same for an oath unaccompanied by an alah? From "and heard the alah" - "and heard the voice" ("of an oath"), equating an oath unaccompanied by an alah to an oath accompanied by an alah.