What is the story of Das Ganu and the holy waters of Prayag mentioned in Chapter 22?

Chapter 22 of the Shri Sai Satcharitra recounts a miraculous story involving the devotee Das Ganu. When Das Ganu sought permission to go on a pilgrimage to bathe in the holy waters of Prayag, Sai Baba offered him an alternative. Baba told him, 'Know this big toe of Mine to be Prayag; bathe right here.' As the text in Chapter 22 narrates, when the faithful Das Ganu placed his head on Baba's feet in acceptance, a miracle occurred. The sacred waters of both the Ganga and Yamuna rivers, which famously converge at Prayag, manifested right there from Baba's big toe, satisfying Das Ganu and demonstrating that the Sadguru's feet are the holiest of pilgrimage sites.