deb	http://ftp.de.debian.org/debian lenny main
deb-src	http://ftp.de.debian.org/debian lenny main
deb	http://security.debian.org/ lenny/updates main
deb-src	http://security.debian.org/ lenny/updates main

Quelle:

http://channel.debian.de/faq/ch-dpkgundco.html