Customary law stated in a General Assembly resolution - the ICJ judgment in Military and Paramilitary Activities (Nicaragua v USA) 1986 (see passages cited in the book):
http://www.icj-cij.org/docket/index.php?p1=3&p2=3&code=nus&case=70&k=66
And see the Declaration on Principles of International Law Concerning Friendly Relations and Co-operation among States in Accordance with the Charter of the United Nations, UN General Assembly Resolution 2625 (XXV):
http://www.un.org/documents/ga/res/25/ares25.htm
For an ICJ case looking at a customary rule, see Arrest Warrant of 11 April 2000 (Democratic Republic of the Congo v. Belgium)
http://www.icj-cij.org/docket/index.php?p1=3&p2=3&code=cobe&case=121&k=36
Footnote links:
31 http://www.un.org/Overview/rights.html
32 http://www.un.org/documents/ga/res/25/ares25.htm
http://www.oosa.unvienna.org/SpaceLaw/lpos.html