Talmud for Eduyot 2:3
אַף הוּא הֵעִיד עַל כְּפָר שֶׁהָיָה בְצַד יְרוּשָׁלַיִם, וְהָיָה בוֹ זָקֵן אֶחָד וְהָיָה מַלְוֶה לְכָל בְּנֵי הַכְּפָר וְכוֹתֵב בִּכְתַב יָדוֹ וַאֲחֵרִים חוֹתְמִים, וּבָא מַעֲשֶׂה לִפְנֵי חֲכָמִים וְהִתִּירוּ. לְפִי דַרְכְּךָ אַתָּה לָמֵד, שֶׁהָאִשָּׁה כּוֹתֶבֶת אֶת גִּטָּהּ וְהָאִישׁ כּוֹתֵב אֶת שׁוֹבְרוֹ, שֶׁאֵין קִיּוּם הַגֵּט אֶלָּא בְחוֹתְמָיו. וְעַל מַחַט שֶׁנִּמְצֵאת בַּבָּשָׂר, שֶׁהַסַּכִּין וְהַיָּדַיִם טְהוֹרוֹת, וְהַבָּשָׂר טָמֵא. וְאִם נִמְצֵאת בַּפֶּרֶשׁ, הַכֹּל טָהוֹר:
He, too, [R. Chanina, the adjutant high-priest,] testified about a small village near Jerusalem where there was an old man who lent (money) to all the men of the village and wrote with his own hand a note of indebtedness [on the borrower], and others signed [i.e., kosher witnesses signed the note]; and it (a case of such a note) came before the sages and they permitted it, [even though the writer of the note was the lender and an "interested party."] Correspondingly, you learn that a woman writes her get (bill of divorce) [and kosher witnesses sign], and a man writes his receipt, [a note of "waiving," his wife waiving her kethubah payments.] For there is no authorization of a get except by its signatures [i.e., the witnesses signed on the get are the essential cause of the confirmation of the get. Therefore, when kosher witnesses sign it, it is kosher, even though it is written by the woman.]