Maseches Sukkah, Chapter 3, Mishnah 4. This Mishnah deals with the number of each of the four species - how many of each one must be taken - as well as with the status of hadassim whose tops have been broken off.
The View of Rabbi Yishmael - The Number of Each Species:
"Rabbi Yishmael omer: sheloshah hadassim u'shtei aravos, lulav echad v'esrog echad" - Rabbi Yishmael says: three hadassim and two aravos, one lulav and one esrog. And this is our practice in halachah:
Hadassim - three.
Aravos - two.
Lulav - one.
Esrog - one.
The Law of Broken Hadassim:
Regarding the hadassim, the Mishnah goes into detail: "afilu shenayim ketumim v'echad eino katum" - even two of them broken and one not broken. According to Rabbi Yishmael, even if the tops of two of the three hadassim are broken off, and only one of them is intact, the set is kosher.
"Rabbi Tarfon omer: afilu sheloshtan ketumim" - Rabbi Tarfon says: even all three of them broken. Rabbi Tarfon is more lenient and permits the hadassim even when the tops of all three are broken off, since in his view there is no requirement of hadar, of the natural beauty of the hadas.
The View of Rabbi Akiva:
Rabbi Akiva disagrees with the very premise of Rabbi Yishmael concerning the number of each species: "keshem she'lulav echad v'esrog echad, kach hadas echad va'aravah achas" - just as there is one lulav and one esrog, so too there is one hadas and one aravah. A single hadas and a single aravah suffice.
In practice: Regarding the number of each species, we rule like Rabbi Yishmael - three hadassim and two aravos. And although Mishnah 2 mentions the law of a hadas whose top was broken off, in practice we rule like Rabbi Akiva, that even if the tops of the hadassim were broken off, they remain kosher.