Who does the preface quote to emphasize the importance of serving saints for knowledge?

The preface quotes Lord Samarth Sadguru Dnyaneshwar Maharaj from his commentary on the Shrimad Bhagavad Gita, known as Shri Dnyaneshwari. As described in Chapter 0, Maharaj advises that to gain knowledge, one must serve the saints with their entire being, surrendering with body, mind, and soul at their feet. He poetically states that service is the "doorway" to the "treasure of knowledge" and that by performing this service without ego, one's questions will be answered in a way that enlightens the heart and removes all doubt.