CRJ900 First Officer reported a low altitude alert from ATC while on final approach.

Narrative

We were cleared for a visual approach backed up with an ILS for XXL into ZZZ. While intercepting the final approach course and between the initial approach fix and the final approach fix; we descended down to 2700 feet from 4000 feet. We joined the final approach course; and about a mile away from FAF ATC radioed us with an altitude alert. We verified that we were level at 2700 and made sure there were no obstacles depicted on our approach plate. We also visually inspected the area for any obstacles in the vicinity that maybe weren't depicted. The aircraft gave us no warning or advisory that we were low altitude; and we determined there were no obstacles in our path so we continued the approach to land uneventfully.Cause: ATC informed us of a low altitude alert while level at 2700.Suggestion: Smooth out descent so as to cross the final approach fix at the designated altitude