daf bit: Bava Batra 144

The mishna teaches: if someone died and left both adult and minor sons, and the adult sons improved the estate (before it was divided), then they improved it for the common good. If, however, they said to the minor sons (in the presence of witnesses) "we desire to cultivate our own shares and to enjoy the proceeds", then the proceeds belong to them. Similarly, if the widow makes improvements to the estate then they are for the common good, but if she said to the sons "I desire to cultivate my share and enjoy the benefits", the proceeds belong to her. The g'mara then objects to this last part, asking "what does a wife have to do with the property of orphans?" Either she receives her ketubah, a fixed payment, or she looks after the property in exchange for being maintained out of it -- either way she does not inherit! R' Yirmiyahu answers that this is talking about the case where the husband named her an heir along with the children (which he's allowed to do; it's just not the default). (143b mishna, 144a g'mara)