Mishnayos Taanis Perek 3 Mishnah 9

The mishna teaches another halakha with regard to fast days: If they were fasting for rain, and rain fell for them before sunrise, they need not complete their fast until the evening. However, if it fell after sunrise, they must complete their fast. Rabbi Eliezer says: If rain fell before midday, they need not complete their fast; but if it rains after midday, they must complete their fast. The mishna relates: An incident occurred in which the court decreed a fast in Lod due to a lack of rain, and rain fell for them before midday. Rabbi Tarfon said to the people: Go out, and eat, and drink, and treat this day as a Festival. And they went out, and ate, and drank, and treated the day as a Festival, and in the afternoon they came to the synagogue and recited the great hallel, to thank God for answering their prayers.

הָיוּ מִתְעַנִּין וְיָרְדוּ לָהֶם גְּשָׁמִים קֹדֶם הָנֵץ הַחַמָּה, לֹא יַשְׁלִימוּ. לְאַחַר הָנֵץ הַחַמָּה, יַשְׁלִימוּ. רַבִּי אֱלִיעֶזֶר אוֹמֵר, קֹדֶם חֲצוֹת לֹא יַשְׁלִימוּ, לְאַחַר חֲצוֹת יַשְׁלִימוּ. מַעֲשֶׂה שֶׁגָּזְרוּ תַעֲנִית בְּלוֹד, וְיָרְדוּ לָהֶם גְּשָׁמִים קֹדֶם חֲצוֹת. אָמַר לָהֶם רַבִּי טַרְפוֹן, צְאוּ וְאִכְלוּ וּשְׁתוּ וַעֲשׂוּ יוֹם טוֹב. וְיָצְאוּ וְאָכְלוּ וְשָׁתוּ וְעָשׂוּ יוֹם טוֹב, וּבָאוּ בֵּין הָעַרְבַּיִם וְקָרְאוּ הַלֵּל הַגָּדוֹל: