Hi,
since this is a very common question I thought it might be worth giving a general solution.
Code:
% apt-file search --fixed-string /usr/bin/ar
binutils: /usr/bin/ar
binutils-multiarch: /usr/bin/ar
apt-file search --fixed-string /usr/bin/ar 9.48s user 0.76s system 109% cpu 9.340 total
HTH,
Evo2.