Reference for Taanit 3:9
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:
If, while they are fasting, rain should fall before sunrise, they shall not continue to fast the whole day; but they must if after sunrise. Rabbi Eleazar says, "If [it rains] before noon they need not continue to fast the whole day; but they must if the rain commenced after noon is passed." It happened once that a fast [for rain] was ordered in Lod and it rained before noon; when Rabbi Tarfon said unto them, "Go, eat and drink, and make a feast." They went, eat and drank, and made a feast; but in the evening they returned, and sang the great Hallel. (Ps. 136, &c.)