In general, words have great power. Therefore, one should always try to avoid any and all frivolous words and attempt to only speak about matters pertaining to Torah and mitzvos. In particular, the words of tzadik are especially powerful and can accomplish a tremendous amount.
It is related that Rav Hirsch, the Magid of Voidaslov zy”a (the father of Rav Bunim of Peshischa zy”a) once was in Lublin. He needed to send some money to his family in Voidaslov, so he entered the Chozeh’s of Lublin’s bais medrash and asked the men there, “Who is traveling to Voidaslov?”
One of those present at the time was Rav Dovid of Lelov zy”a. He told Rav Hirsch, “I am going back to Lelov. On the way, I can stop in Voidaslov.” The Magid gave him the money and asked him to drop it off by his house. When Rav Dovid got there, he knocked on the door and handed over the money. On his way out, he blessed the family by saying, “Zei gezunt. (Be healthy.)”
A while later, the Magid returned home to Voidaslov. He arrived very late at night, after his entire family had gone to sleep. The door was locked and they did not hear him knocking. However, he had one daughter who was crippled and could not walk, who slept closest to the door. Eventually, she heard the knocking, stood on her feet and walked over to open the door.
When the Magid saw his daughter walking, he couldn’t believe it. He asked her, “When did this miracle occur? How are you able to walk?” She told him that when Rav Dovid brought them money, he blessed them to be healthy. Since then, she had been able to walk.
