There is no Bequest for an Heir
My father wrote (in his will) that a farm was to be owned by his son after him. Bearing in mind that he has four daughters in addition to this son, is it permissible for him to do that? And if the farm was divided up between the son and the four daughters, how would the division be?
Allaah, the Almighty, the All-Powerful has explained in His Book how the estate of the deceased is divided up, saying: Allaah commands you as regards your childrens (inheritance): to the male, a portion equal to that of two females.  The Prophet (sallallaahu alaihi wa sallam) said: Verily Allaah gave every deserving person his right, so there is no bequest for the heir.  According to this, the bequest of this father to his son is an invalid one and it is not permissible to implement it - unless all of heirs agree to this. In that case, there is no objection. If they do not agree, then this farm must be returned to the inheritance and be divided amongst the heirs, as Allaah, the Almighty, the All-Powerful has commanded, and the division must be among all of the heirs. If he has no heirs aside from this son and these daughters, then it (the inheritance) will be divided between them, for the male a portion equal to that of two females. So the value of this farm and whatever else the deceased has left must be divided into shares, for each daughter one share and for each son two shares.
 An-Nisa 4:11  Abu Dawud no. 2870.
Shaykh Muhammad bin Saalih al-`Uthaymeen
Fatawa Islamiyah Vol. 5 Page 61