Article 38(1) of the Statute of the International Court of Justice tells the court to decide cases before it by applying
"(a) international conventions, ....
(b) international custom, as evidence of a general practice accepted as law;
(c) the general principles of law recognised by civilised nations
(d).... judicial decisions and the teachings of the most highly qualified ... as subsidiary means for the determination of the rules of law."
International custom requires both state practice and proof of opinio juris sive necessitatis ie that the state had felt legally obliged to act in that way.
G. Gaja, ‘General Principles of Law’, in R. Wolfrum (ed.), Max Planck Encyclopedia of International Law
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Customary law, International