Besides the worship of Vitthal, what other instances show that Sai Baba required 'firm faith' from his devotees to reveal his divine nature?

Sai Baba consistently taught that firm faith was the key to experiencing his divine power, extending beyond the context of Vitthal worship. A powerful example is found in the story of Das Ganu, who wished to bathe in the holy Prayag. As described in chapter 4, Baba dissuaded him from traveling, stating, "This itself is our Prayag; keep firm faith in your mind." To prove his point, a miraculous event occurred: when Das Ganu bowed at Baba's feet, holy water from the Ganga and Yamuna began to drip from Baba's toes. This leela, witnessed by an emotional Das Ganu, showed that the merits of a pilgrimage could be attained right there in Shirdi, but only if one possessed unwavering faith in Baba's words and divine capabilities.