The datasheet says it's 100Mohms typical. However, that's not the whole story, because when you switch the multiplexer to select the analog input you want, the sample capacitor has to charge up. The code for analogRead() allows no time for this (stupidly, IMO) which is why you often see advice to read from the port twice and discard the first reading. My own measurements indicate that a 10us delay between switching the mux and starting the ADC conversion gives reliable results with up to 100K source resistance. The conversion itself takes around 100us, so an extra 10us isn't very significant.