Hadith.5222 - Al-Bazanti narrated from Abdullah ibn Sinan from Abu Abdullah (as) regarding a man who struck another man on the head with a stick, causing his speech to become impaired.
Imam (as) said
"He should be tested with the letters of the Arabic alphabet. For the letters he can articulate clearly, there is no liability. But for the letters he cannot pronounce clearly, there is blood money (diyyah) due for each, and the total is twenty-eight letters."