  We report the earliest-ever detection of optical polarization from a GRB
forward shock (GRB 141220A), measured $129.5-204.3\,$s after the burst using
the multi-colour RINGO3 optical polarimeter on the 2-m fully autonomous robotic
Liverpool Telescope. The temporal decay gradient of the optical light curves
from $86\,$s to $\sim 2200\,$s post-burst is typical of classical forward
shocks with $\alpha = 1.091 \pm 0.008$. The low optical polarization $P_{BV} =
2.8_{- 1.6} ^{+ 2.0} \, \%$ (2$\sigma$) at mean time $\sim 168\,$s post-burst
is compatible with being induced by the host galaxy dust ($A_{V, {\rm HG}}=
0.71 \pm 0.15 \,$mag), leaving low polarization intrinsic to the GRB emission
itself -- as theoretically predicted for forward shocks and consistent with
previous detections of low degrees of optical polarization in GRB afterglows
observed hours to days after the burst. The current sample of early-time
polarization data from forward shocks suggests polarization from (a) the
Galactic and host galaxy dust properties (i.e. $P \sim 1\%-3\%$), (b)
contribution from a polarized reverse shock (GRB deceleration time, jet
magnetization) or (c) forward shock intrinsic polarization (i.e. $P \leq 2\%$),
which depends on the magnetic field coherence length scale and the size of the
observable emitting region (burst energetics, circumburst density).
