Sa'id bin al-Musayyab (may Allah have mercy on him) said: This verse, "And for each We have appointed heirs of what parents and near relatives leave" [Surah al-Nisa': 33], was revealed about those people who used to adopt men other than their own sons and make them their heirs. Allah Almighty revealed this verse and ordained that something be set aside for them by way of bequest. Allah Almighty returned the inheritance to the mawali, to the blood relatives and to the 'asabah (agnatic heirs), and refused that an inheritance be appointed for adopted sons, or for those who attribute themselves (to someone), but He set a share for them in the bequest. And the conditions they used to lay down regarding inheritance, Allah Almighty rejected them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12531