Bava Batra — Daf 108b

ועוד כדכתיב (במדבר כז, ח) איש כי ימות ובן אין לו

ותנא איידי דאתיא ליה מדרשא חביבא ליה

ומאי דרשא דתניא שארו זה האב מלמד שהאב קודם לאחין יכול יהא קודם לבן ת"ל (במדבר כז, יא) הקרוב קרוב קרוב קודם

ומה ראית לרבות את הבן ולהוציא את האח מרבה אני את הבן שכן קם תחת אביו ליעדה ולשדה אחוזה

אדרבה מרבה אני את האח שכן קם תחת אחיו ליבום כלום יש יבום אלא במקום שאין בן הא במקום שיש בן אין יבום

טעמא דאיכא האי פירכא הא לאו הכי הוה אמינא אח עדיף תיפוק ליה

and, secondly,  [one should follow the order of the Torah,] as it is written, If a man die and have no son?  — The Tanna prefers  [to begin with the case of a father who is heir to his son] because this [law] has been arrived at through an exposition. What is the exposition? — It has been taught: His kinsman,  refers to the [dead man's] father. This teaches that a father takes precedence  over brothers. One might [assume] that he also takes precedence over a son, [therefore] it was expressly stated, that is next [to him],  [which implies] he who is nearest  takes precedence. What reason is there  for including the son  and excluding the brother? — The son is included because, as is known,  he is [entitled] to take his father's place in designating [the Hebrew handmaid of his father to be his wife],  and [also in the redeeming] of a field of [his father's] possession.  On the contrary! [Rather say:] 'The brother is included because he also takes the place of his brother in the case of a levirate marriage.'  Surely levirate marriage only takes place where there is no son, but where there is a son there is no levirate marriage. [From what has been said it appears] that the [only] reason [for the precedence of a son is] that there is this reply,  but had it not [been] so, it would have been held [that] a brother takes precedence, [but cannot] this [law]  be deduced