What is the authenticity of the widely circulated story about Imam Malik, in which he refused to drop the qisas (retribution) penalty for the brother who threw his brother into the well, despite the parents' intercession?

In the Maliki madhhab (school of thought), there is no pardon for assassination (qatl al-ghilah), neither from the victim, nor the guardians, nor the Imam, because the right in it belongs to Allah Almighty. Its apparent meaning is that it cannot be pardoned even if the killer comes in repentance, and even if the victim was a disbeliever and the killer was a free Muslim. Ibn Juzayy and Ibn al-Qayyim concurred with this, citing as evidence the action of the Prophet (peace be upon him) in the story of al-Harith ibn Suwayd, and the action of Umar ibn al-Khattab (may Allah be pleased with him) when he killed five or seven individuals for having assassinated one person.