This is caligraphy by Chen Tuan 陳摶(also known as Chen Xi Yi 陳希夷, Chen Tu Nan 陳圖南, Fu Yao Zi 扶搖子, Bai Yun Xian Sheng 白雲先生) who was born at the end of the Tang Dynasty. During the "five generations" period, he became a hermit in Hua Shan 華山practicing Daoism. He fed solely on air and slept continuously for 100 days. He met the founder of the Song Dynasty long before the latter became emperor. One day they played Wei Qi 圍棋 and the bet was the mountain Hua Shan. He won and when Song Tai Zu 宋太祖was coronated as emperor, he remembered his bet and waived the tax on Hua Shan.
Chen Tuan was so famous that when Zi Wei Dou Shu 紫微斗數 was compiled in writing, his name was on it as the author. This was the usual practice in ancient China and it created a lot of trouble for scholars later to determine who the author really was.