  Magnetic fields play an important role in the evolution of interstellar
medium and star formation. As the only direct probe of interstellar field
strength, credible Zeeman measurements remain sparse due to the lack of
suitable Zeeman probes, particularly for cold, molecular gas. Here we report
the detection of a magnetic field of $+$3.8 $\pm$ 0.3 $\mu$G through the HI
narrow self-absorption (HINSA) toward L1544, a well-studied prototypical
prestellar core in an early transition between starless and protostellar phases
characterized by high central number density and low central temperature. A
combined analysis of the Zeeman measurements of quasar HI absorption, HI
emission, OH emission, and HINSA reveals a coherent magnetic field from the
atomic cold neutral medium (CNM) to the molecular envelope. The molecular
envelope traced by HINSA is found to be magnetically supercritical, with a
field strength comparable to that of the surrounding diffuse, magnetically
subcritical CNM despite a large increase in density. The reduction of the
magnetic flux relative to the mass, necessary for star formation, thus seems to
have already happened during the transition from the diffuse CNM to the
molecular gas traced by HINSA, earlier than envisioned in the classical picture
where magnetically supercritical cores capable of collapsing into stars form
out of magnetically subcritical envelopes.
