import keras.backend as K
import numpy as np
A = np.random.rand(10,500)
B = np.random.rand(500,6000)

x = K.variable(value=A)
y = K.variable(value=B)

z = K.dot(x,y)

# Here you need to use K.eval() instead of z.eval() because this uses the backend session
K.eval(z)