Did Fa'an's virtues and his people's hearts come from Zuo Zhuan?

No. According to Baidu Encyclopedia, the sentence "The law secures the world and virtue nourishes the people" comes from "The Analects of Confucius for Government" by the ancient Chinese thinker Confucius. Confucius believed that only through the combination of governing the country by law and governing the country by virtue can we achieve social stability and people's peace and contentment. In the path of socialist rule of law with Chinese characteristics, adhering to the combination of governing the country by law and governing the country by virtue is an important feature.