Not even Apollo keeps his bow always at full stretch.
In other words, even Apollo has to relax every once in a while! You can find the words in the Roman poet Horace: Neque semper arcum tendit Apollo.
The source for the proverb is David Crystal's book, As They Say in Zanzibar: Proverbial Wisdom from Around the World (Oxford University Press: 2006). The poster is made with AutoMotivator. The image is by Stanisław Wyspiański. The Latin original is: Non semper arcum tendit Apollo.