Pesachim, Chapter 6, Mishnah 3. Sacrifices can be divided into two categories: obligatory sacrifices and voluntary sacrifices.
Obligatory sacrifice - A sacrifice that one is obligated to bring once the cause for it arises: a chatas and an asham as described, as well as the Pesach sacrifice and the like.
Voluntary sacrifice - An optional sacrifice, which comes in two types: an olah and shelamim. A person is permitted to bring an olah or shelamim ten times a day, as many times as he wishes. This is completely optional.
In the first Mishnah of the chapter, we addressed the question: What is the halachah regarding an asham taluy that was designated, and after the designation it became clear to the person that he does not need to bring it, because he found out with certainty whether he committed the transgression or not? Rabbi Eliezer's opinion there was "Yikrav" - he should bring it regardless, "she'im eino ba al chet zeh, harei hu ba al chet acher" - even though it is no longer needed for that specific sin whose facts have become clear, it comes for another sin.
Three approaches regarding the definition of the asham taluy:
Rabbi Eliezer's opinion: The asham taluy is a completely voluntary sacrifice. Therefore, a person may bring it as much as he wishes and as frequently as he wishes, to atone for any sin he might have committed.
The Sages' opinion, and the halachah: The asham taluy is an obligatory sacrifice like all obligatory sacrifices, like a regular asham and a chatas. A person does not bring it simply because he wants to, but only when a halachic obligation is imposed upon him to bring it, and if not - he does not bring it.
A middle approach: The sacrifice is brought voluntarily, but on the condition that there is a reasonable concern that he might be required to bring it as atonement for a sin recorded against him. If it is almost certain that he has no sin - he cannot bring it. It turns out that it is brought voluntarily, but within certain limits: On the day after Yom Kippur he may not bring it, for he has just achieved atonement; and similarly, he may not bring more than one sacrifice a day, since it is unreasonable that he committed a new sin five minutes after bringing the previous asham taluy.
The text of the Mishnah:
"Rabbi Eliezer omer, misnadev adam asham taluy bechol yom u'vechol sha'ah she'yirtzeh" - a person volunteers and brings an asham taluy every single day, meaning every day on which individual sacrifices can be offered, and not on Shabbos or Yom Tov. "U'vechol sha'ah she'yirtzeh" - even throughout the entire day, and even ten times a day, since according to his opinion this is a completely voluntary offering, with no difference from an olah or shelamim.
"Ve'nikra asham chasidim" - these additional asham taluy sacrifices, which are brought voluntarily, are called an asham of the pious. A chasid is one who goes beyond the letter of the law, and this person is not obligated by the letter of the law to bring an asham, but due to a concern of some sin, he brings an additional asham taluy just to be safe.
"Amru alav al Bava ben Buta" - the Tanna Bava ben Buta, "she'hayah misnadev asham taluy bechol yom" - would bring an asham taluy every single day, "chutz me'achar Yom Hakippurim yom echad" - except for the one day immediately after Yom Kippur, for on that day he had just achieved atonement through Yom Kippur, and he did not need to bring it.
And Bava ben Buta said: "Hama'on hazeh" - an oath by this Temple, "ilu hayu manichim li, hayisi mevi" - if the Sages in charge of bringing sacrifices to the Temple had allowed him, he would have brought it even on the day after Yom Kippur, for his opinion is like Rabbi Eliezer that it is completely voluntary: It can be brought whenever one wants and as much as one wants, without the need for a reasonable and specific concern regarding a particular sin, but merely to be on the safe side.
"Ela omrim li, hamten ad she'tikanes lesafek" - the Sages in charge told him that he is not permitted to bring it every day and at all hours of the day, nor on the day after Yom Kippur. Rather, he may bring it once a day if he wishes, in case he committed a transgression - up to this point they permit it, for a voluntary offering is within these boundaries; but not on the day after Yom Kippur, since it is unreasonable that he requires atonement on the day following Yom Kippur.
"Vachachamim omrim, ein mevi'in asham taluy ela al davar she'zedono kares ve'shiggaso chatas" - a person only brings an asham taluy if he did an act that, had he certainly done it, he would be obligated to bring a chatas sacrifice for it: a matter for which one who commits it intentionally is liable to kares, and unintentionally brings a chatas, and here he does not even know if he did it, as described elsewhere in the chapter. And the halachah, as mentioned, follows the Sages.
In summary: In this Mishnah, the Tannaim disagreed regarding the definition of the asham taluy - whether it is a voluntary sacrifice or an obligatory sacrifice. According to Rabbi Eliezer, it is completely voluntary, and a person may volunteer it every day and at any time, and it is called an asham of the pious; Bava ben Buta acted this way every day, except for the day after Yom Kippur, and he said that if they had let him, he would have brought it even then, except that they told him "wait until you enter into a doubt"; and according to the Sages, which is the halachah, one only brings an asham taluy for an offense whose intentional violation carries kares and whose unintentional violation requires a chatas.