It's from the Osiris myth where every piece (14 in total) of his scattered body was found with the exception of his phallus, which had been eaten by a fish.
"Rome, the capital of a great monarchy, was incessantly filled with subjects and strangers from every part of the world, who all introduced and enjoyed the favorite superstitions of their native country. Every city in the empire was justified in maintaining the purity of its ancient ceremonies; and the Roman senate, using the common privilege, sometimes interposed, to check this inundation of foreign rites.
The Egyptian superstition, of all the most contemptible and abject, was frequently prohibited; the temples of Serapis and Isis demolished and their worshipers banished from Rome and Italy. But the zeal of fanaticism prevailed over the cold and feeble efforts of policy. The exiles returned, the proselytes multiplied, the temples were restored with increasing splendor, and Isis and Serapis at length assumed their place among the Roman deities.
From Edward Gibbon - The Decline and Fall of the Rome Empire.