From
白居易《
放言五首·
其三》 (Bai Juyi, Tang, 815 CE), written in exile:
赠君一法决狐疑,
不用钻龟与祝蓍。
试玉要烧三日满,
辨材须待七年期 (Let me give you a method to settle your doubts — no need to burn turtle shells or cast yarrow stalks. To test jade, burn it three full days; to judge timber, wait seven years). A famous argument that judging character takes time.