From the Diary of St. Faustina (#742). Jesus says:
…I am giving you three ways of exercising mercy toward your neighbor: the first – by deed, the second – by word, the third – by prayer. In these three degrees is contained the fullness of mercy, and it is an unquestionable proof of love for Me. By this means a soul glorifies and pays reverence to My mercy. Yes, the first Sunday after Easter is the Feast of Mercy, but there must also be acts of mercy, and I demand the worship of My mercy through the solemn celebration of the Feast and through the veneration of the image which is painted. By means of this image I shall grant many graces to souls. It is to be a reminder of the demands of My mercy, because even the strongest faith is of no avail without works.