Based on Chapter 40, what was the specific incident that served as the primary motivation for writing this book?

According to the narrative in Chapter 40, the foundational inspiration for writing the Shri Sai Satcharitra was a specific act performed by Sai Baba. The author describes the profound wonder he felt upon witnessing Sai, a saint, setting up a grinding stone, spreading His legs, and grinding grain. This seemingly simple act sparked a "strong desire" in the author's mind to document all of Sai's deeds. The chapter explains that this desire arose from the belief that describing such holy acts would destroy impurities and sins, and that God Himself is pleased when the praises of His devotees are sung.