We present infrared spectroscopy of Nova Cassiopeiae 1993 (V705 Cas) in the period before dust formation. We find strong emission in the first-overtone CO bands before the nova reached maximum light - the earliest detection of a molecule in a nova. Our observations suggest the presence of a relatively cool (<~4500 K), neutral zone in the ejecta, as required by chemical modelling of nova winds. CO formation is likely to have been dominated by the H_2 route rather than by direct radiative association of C and O. We suggest that radiative cooling of the ejected material by CO could play a major role in the grain formation process.