Bekhorot 58B

ונוטל אחד מן האלולין והשאר פטורין, ממה נפשך: אי באחד באלול ראש השנה ⁠— דאלול ותשרי מצטרפין ודאב פטורים, ואי באחד בתשרי ראש השנה ⁠— דאב ואלול מצטרפין דתשרי פטורין.

and he takes one of the animals born in Elul as the tithe, and the remainder are exempt, whichever way you look at it. Rava explains: If the new year for animal tithe is on the 1st of Elul, then the animals born in Elul and Tishrei combine to form a total of 10, and the animals born in Av are exempt, as they are from the previous year and there are less than 10 of them. And if the new year is on the 1st of Tishrei, the animals born in Av and Elul combine to form a total of 10, whereas the animals born in Tishrei are exempt.

מאי אמרת: לצטרפו לגורן אחר? עשירי ודאי אמר רחמנא, ולא ספק.

The Talmud raises a difficulty with regard to the opinion of Rava: What are you saying? Why are the 5 animals born in Tishrei exempt? Let them combine with animals at another time of gathering to form a total of 10. Rather, what is the reason one is not required to tithe these 5 animals born in Tishrei? It is because God states: “And all the tithe of the herd or the flock, whatever passes under the rod, the tenth should be sacred to YHWH” (Leviticus 27:32), from which it is derived that a definite tenth must be tithed, and not an uncertain tenth, i.e., an animal that is not definitely the tenth. If the 1st of Elul is the new year, then the 5 born in Tishrei would have combined with the 5 of Elul and are exempt. Therefore, the obligation in this case is uncertain.