Jerusalem Talmud Shevuot 4.9

משנה: שִׁילַּח בְּיַד עַבְדּוֹ אוֹ שֶׁאָמַר לָהֶן הַנִּתְבָּע מַשְׁבִּיעַ אֲנִי עֲלֵיכֶם שֶׁאִם אַתֶּם יוֹדְעִין לֹו עֵדוּת שֶׁתָּבוֹאוּ וּתְעִידוּהוּ הֲרֵי אֵילּוּ פְטוּרִין עַד שֶׁיִּשְׁמְעוּ מִפִּי הַתּוֹבֵעַ:

MISHNAH: If he sent through his slave or the defendant said to them, “I am putting an oath on you that you shall come and testify for him,” they are not liable unless they hear from the mouth of the claimant.

הלכה: שִׁילַּח בְּיַד עַבְדּוֹ כול׳. אָמַר רִבִּי לָעְזָר. מַה תַלְמוּד לוֹמַר אִם לֹא יַגִּיד וְנָשָׂא עֲווֹנוֹ. לֹא. מִפִּי הַתּוֹבֵעַ. מַתְנִיתִין צְרִיכָה לְמַה דְאָמַר רִבִּי לָעְזָר וּמַה דָמַר רִבִּי לָעְזָר צְרִיכָה לְמַתְנִיתִין. אִילּוּ תַנִּינָן וְלֹא תַנָּא רִבִּי לָעְזָר הֲוִינָן אָמְרִין. יִשְׁמְעוּ מִפִּי הַתּוֹבֵעַ וְלֹא נִשְׁבְּעוּ לַנִּתְבָּע חַיָיבִין. הֲוֵי צוֹרְכָה לְמַה דָמַר רִבִּי לָעְזָר. אִילּוּ תַנָּא רִבִּי לָעְזָר וְלֹא תַנִּינָן הֲוִינָן מָרִין. יִשְׁמְעוּ מִפִּי הַנִּתְבָּע וְלֹא נִשְׁבְּעוּ לַתּוֹבֵעַ חַיָיבִין. הֲוֵי צוֹרְכָה לְמַתְנִיתִין וּצְרִיכָה לְמַה דָמַר רִבִּי לָעְזָר עַד שֶׁיִּשְׁמְעוּ מִפִּי הַתּוֹבֵעַ וְיִשְׁבְּעוּ לַתּוֹבֵעַ.

HALAKHAH: “If he sent through his slave,” etc. Rebbi Eleazar said, why does the verse say, if he does not tell, he has to carry his iniquity? Not, from the mouth of the claimant. Our Mishnah needs what Rebbi Eleazar said, and what Rebbi Eleazar said needs our Mishnah. If we had stated but Rebbi Eleazar had not stated, we would have said that they are liable if they had heard from the claimant but had sworn to the defendant. Therefore what Rebbi Eleazar said is necessary. If Rebbi Eleazar had stated but we had not stated, we would have instructed that they are liable if they heard it from the defendant but had sworn to the claimant. Therefore one needs our Mishnah and needs what Rebbi Eleazar said, only if they heard from the claimant and swore to the claimant.