Taanis, Chapter 3, Mishnah 9. The Mishnah discusses those who were fasting for rain, and rain fell on the very day of the fast - are they required to complete the fast or not?
The View of the Tanna Kamma - Sunrise:
If the rain fell before dawn - they do not complete the fast. Rashi explains the reason: they had not yet begun the fast, and it never took effect upon them.
If the rain fell after sunrise - they must complete the fast.
The View of Rabbi Eliezer - Midday:
Rabbi Eliezer sets a different criterion:
If the rain fell before midday - they do not complete the fast. Rashi explains that midday is the time when most people have already eaten, and therefore the basis of his view is that before midday the fast has not truly taken effect upon them.
If the rain fell after midday - they complete the fast.
The Incident in Lod:
A fast was decreed in Lod, and rain fell for them before midday. Rabbi Tarfon, who held like Rabbi Eliezer that midday is the criterion, said to them: "Tze'u ve'ichlu ushesu va'asu yom tov" - go out, eat and drink, and make it a festive day. And indeed they went out, ate and drank and made it a festive day, and did not complete the fast.
In the afternoon hours they came to the synagogue and recited Hallel HaGadol - "Hodu laHashem ki tov ki le'olam chasdo" - give thanks to Hashem for He is good, for His kindness endures forever, chapter 136 in Tehillim - because it states there that the Holy One gives bread to all flesh. The Gemara notes precisely that they did not recite Hallel HaGadol before they ate, for Hallel HaGadol is recited only with a satisfied soul and a full stomach.
In summary: In this Mishnah we learned the dispute over the point in the day from which the fast takes effect and obligates completion - according to the Tanna Kamma sunrise is the dividing line, and according to Rabbi Eliezer it is midday. The incident in Lod illustrates Rabbi Eliezer's view in practice, and teaches that the recitation of Hallel HaGadol over the falling of rain comes specifically out of a state of satisfaction.