By the end of the story, is there any source of hope? What message (if any) does Voltaire leave the reader with?

The answer to this is unclear, because Voltaire's Candide is such a pessimistic work overall. It is very difficult to find much optimism within it, and the main character, who seems at times to be the incarnation of optimism and hope itself, is torn down until he loses his own positive attitude. However, the end of the book gives something of a challenge, which offers hope in the context of the work.
Optimism is clearly the enemy in Voltaire's eyes. He embraces cynicism and encourages his main character to eventually accept that the world is not necessarily a happy and optimistic place. Most prominently, he seems to be decrying the majority of European civilization and the world at large, blaming humanity for the state of the world (which would not be an unfair assessment).
After seeing wickedness and corruption (e.g., the Portuguese Inquisition, thieves and cannibals trying to take both his possessions and his life, admirals and soldiers who abuse and fight with him, people being quartered and tortured, and so much more), Candide has experienced the worst humanity has to offer. He receives a shimmering gleam of hope, however, when he enters El Dorado and finds a land of peace and intelligence, without the violence, religious turmoil, or bitterness inherent in the rest of the land. But he unfortunately departs and loses everything he has gained in that experience.
Following all this, Candide is eventually overcome by despair. However, the one hopeful sign in the book comes at the very end, when Candide and his companions gather together on the farm that he purchases with the remnants of the finances acquired during his travels, and they set to work on their own version of civilization. The group focuses on their own talents and endeavors to work together, in a situation that resembles communism, and Candide states at the very end that "we must cultivate our own garden." This final line is the one area of optimism and hope—by working hard and eschewing the evils of society, it implies, we can make a better place in the future.