As discussed in Mitzvah #91, there is a mitzvah to bring first fruits to the Temple. When doing so, one had to recite a passage from Deuteronomy 26:5-10, the section of “Arami oveid avi” (“an Aramean tried to destroy my ancestor” or, possibly, “my ancestor was a wandering Aramean.” This passage may be familiar from the Passover Seder).
The reason for this mitzvah is because a person is inspired by what comes out of his mouth. By speaking the declaration aloud, the message is impressed upon him. The landowner doesn't only bring his first fruits to the Temple in order to share the good things that God has given him, he verbally acknowledges God's goodness. The passage describes how God saved the patriarch Jacob from Laban, redeemed the Jews from Egypt and brought them into Israel. It culminates with the personal goodness with which God has graced the person making the declaration, as demonstrated through his first fruits.
If one delivers first fruits on behalf of another person, he does not make the declaration. This is because the last line of the declaration says "that God has given me" and God did not give it to the messenger, He gave it to the one who sent the messenger. Even though the overwhelming majority of the passage applies to all Jews, a messenger does not recite it because of one word that is not accurate in his case. From this, the Sefer HaChinuch infers a lesson in how much caution we should exercise in our prayers.
This mitzvah applies in Temple times. It is discussed in the Talmud in tractate Sotah (32a-b) and in the Mishna in the third chapter of tractate Bikkurim. It is codified in the Mishneh Torah in the third chapter of Hilchos Bikkurim. This mitzvah is #132 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos.