The first-year Wilkinson Microwave Anisotropy Probe (WMAP) data, in combination with any one of a number of other cosmic probes, show that we live in a flat lambda -dominated cold dark matter (CDM) universe with Omega sub(m) ~ 0.27 and Omega sub( lambda ) ~ 0.73. In this model the late- time action of the dark energy, through the integrated Sachs-Wolfe effect, should produce cosmic microwave background (CMB) anisotropies correlated with matter density fluctuations at z ~ 2 (Crittenden & Turok 1996). The measurement of such a signal is an important independent check of the model. We cross-correlate the NRAO VLA Sky Survey (NVSS) radio source catalog (Condon et al. 1998) with the WMAP data in search of this signal, and see indications of the expected correlation. Assuming a flat lambda CDM cosmology, we find Omega sub( lambda ) > 0 (95% CL, statistical errors only) with the peak of the likelihood at Omega sub( lambda ) = 0.68, consistent with the preferred WMAP value. A closed model with Omega sub(m) = 1.28, h = 0.33, and no dark energy component (Omega sub( lambda ) = 0), marginally consistent with the WMAP CMB TT angular power spectrum, would produce an anticorrelation between the matter distribution and the CMB. Our analysis of the cross-correlation of the WMAP data with the NVSS catalog rejects this cosmology at the 3 capital sigma level.