The decay is observed using data corresponding to an integrated luminosity of 3.0 fb−1 recorded by the LHCb experiment. The decay proceeds at leading order via a loop transition and is therefore sensitive to the possible presence of particles beyond the Standard Model. A first observation is reported with a significance of 5.9 standard deviations. The value of the branching fraction is measured to be i5" class="mathmlsrc">i5.gif&_user=111111111&_pii=S0370269316302325&_rdoc=1&_issn=03702693&md5=69bac0455e71c4dce989969e9456293b">i5.gif">, where the first uncertainty is statistical, the second is systematic, and the third is related to external inputs. Triple-product asymmetries are measured to be consistent with zero.