The pupils of R. Ishmael dissect a prostitute, who had been condemned to death and had died, and find 252 bones in her and 243 in man. [Then follows in text an enumeration of the 253 bones in the human body.]
The Students of Ishmael Count the Bones
Gaster, Exempla of the Rabbis, No. 231; Talmud, Bekhorot 45a