CERN-LHC. A search for a standard model Higgs boson produced in association with a top-quark pair and decaying to bottom quarks is presented. Events with one charged lepton (electron or muon) and at least five jets, or two oppositely charged leptons (electron-electron, muon-muon, electron-muon) and at least four jets, are selected from a data sample corresponding to an integrated luminosity of 19.5 invfb collected by the CMS experiment at the LHC in pp collisions at a centre-of-mass energy of 8 TeV. Leptons are defined by: - ptLepton > 30 (20) GeV for the single lepton (dilepton) channel - |etaLepton| < 2.1 (2.5) for muons (electrons) - sum of travserse momentum of particles within dR < 0.4 of the lepton direction < 10% of ptLepton. Jets are defined by clustering particle-flow particles using the anti-kT algorithm with radius R=0.5 and are required to have: - ptJet > 30 GeV - |etaJet| < 2.5. The CSV algorithm is used to assign a b tagging score to each jet. In order to separate the signal from the larger ttbar+jets background, this analysis uses a matrix element method that assigns a probability density value to each reconstructed event under signal or background hypotheses. The ratio between the two values is used in a maximum likelihood fit to extract the signal yield. The results are presented in terms of the measured signal strength modifier, mu, relative to the standard model prediction for a Higgs boson mass of 125 GeV. The observed (expected) exclusion limit at a 95% confidence level is mu < 4.2 (3.3), corresponding to a best fit value of 1.2 +1.6,-1.5.