Here it is:
Praise be to Allaah.
It says in the statements of the Islamic Fiqh Council in Makkah al-Mukarramah, quoting from Fiqh al-Nawaazil by al-Jayzaani (4/208-209):
Based on necessities which call for dissection or autopsy of the dead body and in which dissection or autopsy serves an interest which outweighs the concern about violating the sanctity of the deceased, the Fiqh Council of the Muslim World League has determined the following:
1 – It is permissible to dissect a dead body for one of the following purposes:
(a) Examination in the case of a criminal investigation to find out the causes of death or what crime was committed; that is when the qaadi (judge) is uncertain of the causes of death and thinks that dissection or autopsy is the way to find out these causes.
(b) Investigation of diseases in cases where dissection or autopsy is called for so that in the light of this post mortem examination, precautions may be taken or suitable treatments may be determined for those diseases.
(c) Learning and teaching medicine as is done in schools of medicine.
Source:
http://www.islam-qa.com/en/ref/92820/post mortem