Annex to the Convention: Regulations respecting the laws and customs of war on land - Section I : On belligerents - Chapter II : Prisoners of war - Regulations: Art. 18.
Art. 18. Prisoners of war shall enjoy complete liberty in the exercise of their religion, including attendance at the services of whatever church they may belong to, on the sole condition that they comply with the measures of order and police issued by the military authorities.