Model soldier and martyr Zhang Side towering above mountains and clouds, almost transformed into a mountain himself.

The text is a Mao quotation from his 1944 speech 'Serve the people' at a memorial meeting for Comrade Zhang Side. 
The full text:
人总是要死的,但死的意义有不同。……………为人民利益而死, 就比泰山还重; 替法西斯卖力,替剥削人民和压迫人民的人去死, 就比鸿毛还轻。张思德同志是为人民利益 而死的, 他的死是比泰山还要重的。
As translated in Mao's Selected Works Volume III and the Little Red Book:
All men must die, but death can vary in its significance. To die for the people is weightier than Mount Tai, but to work for the fascists and die for the exploiters and oppressors is lighter than a feather. Comrade Chang Szu-teh died for the people, and his death is indeed weightier than Mount Tai.

The impressum states: 毛主席语录画展览会供稿 - Contributed by Chairman Mao's Quotations Painting Exhibition.