What specific act of Sai Baba is mentioned in Chapter 40 as the original inspiration for writing the Shri Sai Satcharitra?

According to Chapter 40, the root inspiration for the book came from witnessing a seemingly simple act performed by Sai Baba. The author recalls the profound image of Sai, a revered saint, setting up a grinding stone in Shirdi and grinding grain. This sight filled the author's mind with such wonder that a 'strong desire arose in the mind that by describing all such deeds, impurities (sins) would be destroyed.' This chapter explains that the author realized that singing the praises of God's devotees is a powerful form of worship that pleases God immensely.