By giving the man perseverance of faith! Verse 41 is causal with respect to verse 40 (“for”). The reward will be that such a man will be kept from joining the crowd in cursing Jesus. He does not follow with the disciples, but Jesus knows him and will be able, also over a greater distance, to keep this man for God who sent him.1
41 For truly, I say to you, whoever gives you a cup of water to drink because you belong to Christ will by no means lose his reward.