Shevuos, Chapter 4, Mishnah 9. This mishnah deals with another case in which witnesses are exempt from the oath of testimony.
The case of the mishnah:
A person comes to two witnesses and administers an oath to them, that when testimony relevant to him becomes known to them in the future, they will come and testify on his behalf. They are exempt, "mipnei shekadmah shevuah la'edus" - the oath was administered before they actually witnessed the event, and there is no liability except when they swear regarding testimony that they have already seen.
The comment of Rabbi Akiva Eiger:
Rabbi Akiva Eiger infers that the mishnah must be speaking of a case in which the witnesses did not respond at all to the oath, for one must distinguish between two situations:
They responded and accepted the oath upon themselves: One who answers amen is considered as though he pronounced the oath with his own mouth, and therefore they would be obligated to give testimony in the future - not because of the oath of testimony, but because of a shevuas bituy (an oath of expression).
They did not respond at all: This is where the ruling of the mishnah applies, that they are exempt.
It is a general rule that anyone who swears to do a certain thing is obligated to fulfill his oath - and again, not because of the oath of testimony but because of a shevuas bituy. From here comes the proof: since the mishnah exempts the witnesses across the board, it must be speaking of a case in which they did not respond at all to that oath.
In summary: In this mishnah we learned that one who administers an oath to witnesses regarding testimony that will become known to them in the future - they are exempt, because the oath preceded the testimony. Rabbi Akiva Eiger establishes the ruling of the mishnah specifically in a case where the witnesses did not respond at all, for had they answered amen and accepted the oath, they would have been obligated on account of a shevuas bituy.