These are the default entries from an unmodified image:
rpi ~$ cat /etc/apt/sources.list
deb http://raspbian.raspberrypi.org/raspbian/ buster main contrib non-free rpi
# Uncomment line below then 'apt-get update' to enable 'apt-get source'
#deb-src http://raspbian.raspberrypi.org/raspbian/ buster main contrib non-free rpi
rpi ~$ cat /etc/apt/sources.list.d/raspi.list
deb http://archive.raspberrypi.org/debian/ buster main
# Uncomment line below then 'apt-get update' to enable 'apt-get source'
#deb-src http://archive.raspberrypi.org/debian/ buster main
After you restore these values, you will need to run sudo apt update
again.
buster
??stretch
is a possibility, but quite old now.