Mishnayos Shevuos Perek 4 Mishnah 11

If one said to two people: I administer an oath to you, so-and-so and so-and-so, that if you know testimony relevant to me you will come and testify on my behalf, and they respond: On our oath we do not know any testimony relevant to you, and they know testimony relevant to him, not based on an incident they witnessed but based on hearsay testimony, which is not valid testimony, or if one of the witnesses is found to be a relative or disqualified, these witnesses are exempt from liability for taking a false oath of testimony because they are unfit witnesses.

אָמַר לִשְׁנַיִם, מַשְׁבִּיעַ אֲנִי עֲלֵיכֶם אִישׁ פְּלוֹנִי וּפְלוֹנִי, שֶׁאִם אַתֶּם יוֹדְעִין לִי עֵדוּת שֶׁתָּבֹאוּ וּתְעִידוּנִי, שְׁבוּעָה שֶׁאֵין אָנוּ יוֹדְעִין לְךָ עֵדוּת, וְהֵם יוֹדְעִין לוֹ עֵדוּת עֵד מִפִּי עֵד אוֹ שֶׁהָיָה אֶחָד מֵהֶן קָרוֹב אוֹ פָסוּל, הֲרֵי אֵלּוּ פְטוּרִין: