Shulchan Arukh, Orach Chayim 653/שולחן ערוך·אורח חיים תרנ״ג

1It is forbidden to smell the myrtle. Containing 2 paragraphs It is forbidden to smell the myrtle that is used for the mitzvah. Technically, it is permitted to smell the etrog, but because there is a disagreement about whether or not to bless on it, one should refrain from smelling it.

2If [one of the four species] was made for [use as one of the four species] but was not lifted to fulfill an obligation, it is still permissible to get benefit from it.