Sunzi wrote in the Art of War (written in the 6th century BCE):
"Prisoners of war shall be treated well and cared for. This is called using the defeated enemy to increase (our own) strength."
The Southern Song (1127-1279) annotator Zhang Yu commented this passage thus:
"All soldiers taken prisoner must be cared for with magnanimity and good faith, so that they may be be used by us."
Translation of the two passages by me.
Hat tip: The Useless Tree