Albert Barnes on Mark 9:41

Whosoever shall give you a cup, etc. How easy it is to be a Christian! What is easier than to give a cup of cold water to a thirsty disciple of Jesus! But it must be in his name--that is, because he is a Christian, and therefore from love to the Saviour. This is very different from giving it from a mere motive of common kindness. If done from love to Christ, it will be rewarded; and hence we learn that the humblest acts of Christians, the lowest service that is rendered, will be graciously noticed by Jesus, and rewarded. None are so humble in his kingdom as not to be able to do good; and none so poor that, in his circumstances, may not show attach merit to him. Their feeblest service will be accepted; and acts of love, that may be forgotten by man, will be remembered by him, and rewarded in heaven.

(u) "for whosoever" Mt 10:42, 25:40