Yevamot

Some say: As regards intercourse all agree that the intercourse of a younger brother is preferred.  They only differ on the halizah of a younger brother. And the statement  ran thus: [On the relative importance of] the halizah of a younger, and the halizah of an elder brother there is a difference of opinion between R. Johanan and R. Joshua b. Levi. One holds that the halizah of the elder is preferable, and the other holds that both are of equal importance. 'One  holds that the halizah of the elder is preferable'  because the commandment surely, is incumbent upon the elder. And the other [maintains that] the statement, 'the commandment is incumbent upon the elder', [was made] in respect of the levirate marriage; in respect of the halizah, however, they are both of equal importance. We learned, IF THEY ALSO DECLINE, THE ELDEST IS AGAIN APPROACHED. Does not this mean that they declined to contract the levirate marriage but [were willing] to submit to halizah? And yet it was stated, THE ELDEST IS AGAIN APPROACHED, which proves that the halizah of the elder is preferred! — No; they declined the halizah as well as the levirate marriage. Similarly, in the case of the eldest brother, he declined the halizah as well as the levirate marriage;  why, then, IS THE ELDEST AGAIN APPROACHED with the object of coercing him? Let coercion be used against them!  — As the duty  is incumbent upon him, coercion also must be used against him. Come and hear: IF HE WISHES TO SUSPEND ACTION … UNTIL THE ELDEST RETURNS FROM A COUNTRY BEYOND THE SEA … HE IS NOT TO BE LISTENED TO. But if the halizah of the eldest is preferable why IS HE NOT TO BE LISTENED TO? Let us rather wait, since it is possible that when he returns he will submit to halizah! — Following your view [it might similarly be objected], why [if he wishes to postpone action] UNTIL A MINOR BECOMES OF AGE … HE IS NOT TO BE LISTENED TO? Let us rather wait, since, on becoming of age, he might contract the levirate marriage!  The fact is that the performance of a commandment must not be delayed. We learned elsewhere: At first, when the object was the fulfilment  of the commandment, the precept of the levirate marriage was preferable to that of halizah; now, however, when the object is not the fulfilment of the commandment, the precept of halizah, it was laid down, is preferable to that of the levirate marriage.  Rab said: But no coercion  may be used. When they  came before Rab he addressed them thus: 'If you  wish, submit to halizah; if you prefer, contract the levirate marriage; the All Merciful has given you the choice:  And if the man like not to take his brother's wife,  implying, if he likes he may, whenever he wishes, submit to halizah or, if he prefers, contract the levirate marriage.' Rab Judah also is of the opinion that no coercion may be applied; since Rab Judah has ordained [the following formula] for a deed of halizah: '[We certify] that So-and-so daughter of So-and-so brought before us into court her brother-in-law So-and-so, and we have ascertained him to be the paternal brother of the deceased. We told him, "If you wish to contract the levirate marriage, contract it, and if not, incline  towards her your right foot". He inclined  towards her his right foot and she removed his shoe from off his foot and spat out before him, a spittle which has been seen by the court upon the ground'. R. Hiyya b. Iwya in the name of Rab Judah concluded  as follows: 'And we read before them [the relevant passage] that is written in the Book of the Law of Moses'. 'We ascertained him'. On this, R. Aha and Rabina are in dispute. One says: Through [qualified] witnesses. The other says: Even a relative and even a woman  [may tender the evidence]. The law is that it  is a mere intimation, and that even a relative and even a woman [may tender the evidence]. 'At first, when the object was the fulfilment of the commandment, the precept of the levirate marriage was preferable to that of halizah; now, however, when the object is not the fulfilment of the commandment, the precept of halizah, it was laid down, is preferable to that of the levirate marriage'. Said Rami b. Hama in the name of R. Isaac: It was re-enacted that the precept of the levirate marriage is preferable to that of halizah. Said R. Nahman b. Isaac to him: Have the generations improved in their morals? — At first they held the opinion of Abba Saul, and finally they adopted that of the Rabbis. For it was taught: Abba Saul said, 'If a levir marries his sister-in-law on account of her beauty, or in order to gratify his sexual desires or with any other ulterior motive, it is as if he has infringed the law of incest; and I am even inclined to think that the child [of such a union] is a bastard'. But the Sages said, 'Her husband's brother shall go in unto her,  whatever the motive'. Who is the Tanna of the following statement which our Rabbis taught: 'Her husband's brother shall go in unto her,  is a commandment; for originally  she stood in relation to him in the status of permissibility, then  she was forbidden to him, and then again  permitted; consequently it might have been assumed that she reverts to her original status of permissibility, hence it was specifically stated, Her husband's brother shall go in unto her,  it is a commandment'. — Who, now, is the Tanna? — R. Isaac b. Abdimi replied. It is [the statement of] Abba Saul, and it is this that he meant: Her husband's brother shall go in unto her,  is a commandment; for originally  she stood in relation to him in the status of permissibility; he could have married her, if he wished, on account of her beauty and he could have married her, if he wished, in order to gratify his sexual desires; then  she was forbidden to him, and then again  permitted; consequently it might have been assumed that she reverts to her original status of permissibility,  hence it Was specifically stated, Her husband's brother shall go in unto her  only with the intention of performing the commandment. Raba said: You may even say [that the authorship  is that of] the Rabbis,  and it is this that was meant: Her husband's brother shall go in unto her,  is a commandment; for originally  she was in the status of permissibility; he could have married her if he wished and, if he preferred, he could have abstained from marrying her; then  she was forbidden to him, and then again  permitted; consequently it might have been assumed that she was to revert to her original status of permissibility, so that, if he wished, he might marry her and, if he preferred, he could abstain from marrying her. [You say,] 'If he preferred he could abstain from marrying her'? Surely she is tied to him;  can she be set free by no act whatever! — Say rather: [It might have been assumed that] if he wished he might marry her, and, if he preferred, he might submit to halizah, hence it was specifically stated her husband's brother shall go in unto her,  it is a commandment. Read, then,  the first clause: 'It shall be eaten without leaven in a holy place,  is a commandment;