Simhah ben Yehudah ha-Sofer, completed 28 Teveth 5033 (20 December 1272 Julian) - Mahzor Worms (Israel Ms. Heb. 4°781
This stunning page of a festival prayerbook, found in the Machzor of Worms, is the oldest example of an identifiable yiddish sentence. The rhyming couplet is cleverly nestled inside the Hebrew title in red ink.
גוּט טַק אִים בְּטַגְֿא שְ וַיר דִּיש מַחֲזֹור אִין בֵּיתֿ הַכְּנֶסֶתֿ טְרַגְֿא
Gut taq im betage se vaer dis makhazor in beis-hakneses trage.
Let a good day dawn for him who would carry this prayer book to the synagogue.
This little note is directed to someone who recited the books Hebrew benedictions but did not speak Hebrew day to day. The sentence reaches out to the next machzor owner or carrier. Positive, upbeat, practical, and deeply rooted in Jewish history. That's Yiddish.
Here is a link to the National Library of Israel's page devoted to the Worms Machzor.