minimum commit id length when fixing a patch

Hi,
I want to send a patch to LKML fixing something wrong which was caused
by other patch.

Form what I understand, it is a good practice to mention the commit
title and the commit ID.

A commit ID is very long. My question is: when mentioning in my commit
log the patch that caused the problem, what is the minimum commit id
length which I should use when presenting the patch that caused the
problem ? is such commit length is documented anywhere ?

Regards,
Kevin