Before us is Mishnah 6 in chapter 4 of Maseches Yevamos, which deals with a yavam who seeks to put off fulfilling the mitzvah that rests upon him by pinning it on another brother.
The text of the Mishnah:
"HaToleh b'katan bizman sheyagdil, o b'gadol ad sheyavo mimedinas hayam, o b'cheresh o b'shoteh - ein shom'in lo, ela omrim lo: alecha mitzvah, o chalotz o yabem".
The three claims the yavam makes:
"HaToleh b'katan bizman sheyagdil" - the yavam claims that one of the brothers is still a minor, and therefore we should wait until he grows up and see what he says; perhaps he will be the one to perform yibbum.
"O b'gadol ad sheyavo mimedinas hayam" - there is another adult brother, but he is overseas. According to the yavam, we should wait until he returns from overseas; perhaps he will be the one to perform yibbum.
"O b'cheresh o b'shoteh" - he has a brother who is a deaf-mute or a brother who is mentally incompetent, and he claims that perhaps they will recover, perhaps a cure will be found, and the like.
The ruling of the Mishnah:
"Ein shom'in lo" - we do not listen to that brother who says: I will wait until my younger brother grows up, or until my brother returns from abroad, or perhaps my deaf-mute and mentally incompetent brothers will recover.
"Ela omrim lo: alecha mitzvah, o chalotz o yabem" - the mitzvah rests upon you, and you must fulfill it: either perform chalitzah or perform yibbum. We do not wait.
In summary: in this Mishnah we learned that a yavam who pins the fulfillment of the mitzvah on another brother - on a minor who will grow up, on an adult who will return from overseas, or on a deaf-mute and a mentally incompetent brother who may recover - is not listened to. Rather, we obligate him to fulfill what rests upon him himself: chalitzah or yibbum, with no postponement and no waiting.