Yevamot

hence it was explicitly stated, 'As the sin-offering so is the guilt-offering': As the sin-offering  requires application of the blood to, and burning of the prescribed portions upon the altar, so does the guilt-offering also require application of the blood to, and burning of the prescribed portions upon the altar.  Had Scripture not restored it,  however, it would have been assumed that it was singled out only in respect of what was explicitly specified but not in any other respect;  so also here,  I would assume, only a brother's wife who was explicitly mentioned [can be said] to be permitted  but not any of the other forbidden relatives! But  it might have been assumed that the law of a wife's sister  should be deduced from what has been found in the case of a brother's wife; as a levir may marry his brother's wife so he may also marry his wife's sister. Are, however, the two cases  similar? In the one case  there is only one prohibition; in the other  there are two prohibitions!  — It might have been assumed that since she  was permitted  [in respect of one prohibition]  she was also permitted [in the case of the other].  And whence is it derived that we assume that 'since something was permitted [in one respect] it was also permitted [in the other]'? — From what was taught: In the case of a leper whose eighth day [of purification]  fell on the Passover eve,  and who, having observed a discharge of semen on that day,  had taken a ritual bath, the Sages said: Although no other tebul yom  may enter [the Temple mount],  this one  may enter, for it is better that the positive precept,  the non-observance of which involves kareth, shall supersede a positive precept  the infringement of which involves no kareth.  And in connection with this R. Johanan said: According to the Torah, not even [the infringement of] a positive precept is involved,  for it is said, And Jehoshaphat stood in the congregation of Judah … before the new court.  What is meant by the new court? Rabbi  replied: That they enacted therein new laws, ordaining that a tebul yom  must not enter the camp of the Levites.  And 'Ulla said: 'What is the reason?'  Since he was given permission  in respect of his leprosy,  permission was also given to him in respect of his discharge of the semen.  But is this case  similar to that of 'Ulla?