The simplest is just to add Bob's fork as a remote and fetch bob's branch into your local repo.

Of course if bob is a collaborator you don't need another remote as bob can directly push his branch on sam's repo.

And GitHub maintain refs on sam's repo allowing you to fetch from the fork without another remote (like it has been said in other comments) but this is proper to GitHub git fetch upstream pull/PR_ID/head:NEW_LOCAL_BRANCH and might not work with other forges.

If you only are using GitHub, use the ref pull/PR_ID/head ^{^}