Li, originally surnamed Xu and styled Maogong, was granted the imperial surname Li by the Tang emperor as a reward for his great achievements.
In the final years of the Sui Dynasty, Li Mi and Zhai Rang led a rebel army, and a man named Xu Shiji joined them. Later, Li Mi surrendered to the Tang Dynasty but was executed for plotting rebellion. Xu Shiji, never forgetting his former master, donned mourning clothes and buried Li Mi with full honors.
After Emperor Taizong of Tang ascended the throne, he appointed Xu as the governor of Bingzhou, guarding the northern frontier for 16 years. The Turks, awed by Xu's reputation, dared not invade south. Emperor Taizong once proudly said:
"Emperor Yang of Sui did not select suitable talents to guard the borders, but instead labored the people to build the Great Wall to defend against the barbarians. Now I use Xu to guard Bingzhou, which is far stronger than his Great Wall."
Later, Emperor Taizong of Tang appointed his son Li Zhi, Prince of Jin, as the crown prince. Recognizing that Xu had been utterly loyal and dedicated to the court, Taizong believed he could entrust him with state affairs to guide the prince, and thus showed him special care. Once, Xu suddenly fell gravely ill, and a shaman said that ash from whiskers could cure him. Emperor Taizong cut off his own beard, personally mixed it into the medicine, and fed it to Xu. Another time, when the two were drinking together, Xu became heavily intoxicated, and Taizong removed his own robe to drape over Xu. Moved to tears, Xu wept, and Taizong said to him, "I do this not for your sake, but for the future of the empire."
"I want to entrust the crown prince to you; no one is more suitable than you. You never forgot Li Mi back then, so you will not betray me." Later, the idiom "Cutting the Beard to Mix Medicine" came to describe a superior or elder showing care and concern for subordinates or juniors.
Source: *New Book of Tang*, "Biography of Li"
Meaning of the Idiom: Later, the Chinese idiom "剪须和药" came to describe a superior or elder showing care and concern for subordinates or juniors.