"The Messenger of Allah(S.A.W) came to visit me while I was ill at Banu Salamah. I said : 'O Prophet of Allah(S.A.W)! How shall I divide my wealth among my children?' But he did not say anything to me, until the following was revealed: Allah commands you regarding your children's (inheritance): to the male, a portion equal to that of two females."
Jami` at-Tirmidhi Book 29, Hadith 7