Phra Ajaan Sao together with Phra Ajaan Mun are the founders of the tradition of Forest Meditation monks of Thailand.
The revival of the Thai Forest Tradition in the twentieth century was an attempt to return to the lifestyle and training that was practiced under the Buddha. Their intention was to realize in their own hearts and minds the inner peace and wisdom of the Dhamma. The busy village monasteries were abandoned for the peace and quiet of nature. The Vinaya was followed strictly, emphasizing the importance of every detail. Monks lived without money, accepting whatever was offered and patiently enduring when nothing was. Ascetic practices recommended by the Buddha were instituted as part of the lifestyle: eating only one meal a day from one's almsbowl, wearing rag robes, and living in the forest, in cemeteries or in simple shelters.
The monks would often wander barefoot through the countryside seeking places conducive to meditation, carrying their few possessions: an alms bowl, three robes, a glot (an umbrella with a mosquito net, which was hung in the forest and used like a tent), and a few personal requisites.
From Ajaan Sao, Ajaan Mun and their numerous distinguished disciples has come a legacy of powerfully relevant examples of an uncomplicated and disciplined way of life. Their teachings are directed towards those who wish to purify their minds by living the path of practice taught by the Buddha. The very heart of the Forest Tradition is the development of meditation. By cultivating deep states of tranquility and systematically investigating the body and mind, insight arises as to the true nature of existence.
After the cremation of Ajaan Sao, his bone fragments were distributed to people and around the Thai provinces. And according to his followers, they transformed into crystal-like relics (Pali: Sarira-Dhatu) in various hues.
An extract from "Ajaan Sao's Teaching - A Reminiscence of Phra Ajaan Sao Kantasilo" is as follows:
Phra Ajaan Sao was inclined to be, not a preacher or a speaker, but a doer. When he taught his students, he said very little. And those who studied directly under him are now elders who speak very little, who rarely preach, having picked up the habit from their teacher. Thus, as Phra Ajaan Sao was not a preacher, I would like to tell you a little of the way in which he taught meditation.
How did Phra Ajaan Sao teach? If it so happened that someone came to him, saying, "Ajaan, sir, I want to practice meditation. How should I go about it?" he would answer, "Meditate on the word 'Buddho.'"
If the person asked, "What does 'Buddho' mean?" Ajaan Sao would answer, "Don't ask."
"What will happen after I've meditated on 'Buddho'?"
"Don't ask. Your only duty is simply to repeat the word 'Buddho' over and over in your mind."
That's how he taught: no long, drawn-out explanations."Ajaan Sao's Teaching: A Reminiscence of Phra Ajaan Sao Kantasilo", transcribed from a talk by Phra Ajaan Phut Thaniyo, translated from the Thai by Thanissaro Bhikkhu. Access to Insight, June 7, 2010, http://www.accesstoinsight.org/lib/thai/phut/sao.html.