The 248th mitzvah is that we are commanded regarding the laws of inheritance.
The source of this commandment is G‑d's statement1 (exalted be He), "If a man dies and has no son..."
This mitzvah undoubtedly includes the law that the firstborn son inherits a double portion,2 since this is part of the laws of inheritance.
The details of this mitzvah are explained in the 8th and 9th chapters of tractate Bava Basra.
