I heard a story about one of Sai Baba's devotees being attacked by robbers. What happened during that incident?

You are likely referring to the devotee Kashiram. According to Chapter 0, while returning from a market, Kashiram was accosted by Bhil robbers. He did not object when they took some of his belongings. However, when they reached for a small, precious bundle he carried, he decided he would not let it go. In the ensuing struggle, a thief dropped his sword, which Kashiram picked up and used to kill two of them. A third thief then struck him on the head with an axe, and he fell unconscious. The robbers, thinking he was dead, left him. He survived and insisted on being taken to Shirdi for recovery due to his faith in Maharaj.