Conversation so far:
What lead up to Umar's death?
On his deathbed, Umar vacillated on his succession. Umar finally appointed a committee of six persons to choose a caliph from amongst them.
What was usamas army doing while Umar vacillated on his succession?
The only 'famous ten' left out of the committee who was still alive at the time was Saeed ibn Zaid, the cousin and brother-in-law of Umar.
Are there any other interesting aspects about the aftermath after Umar's assassination besides choosing a committee for the next caliph and leaving Saeed ibn Zaid, his cousin and brother-in-law, out of the decision?
It seemed clear that Umar's assassination had been planned by Persians residing in Medina. Infuriated by this, Umar's younger son Ubaidullah ibn Umar sought to kill all Persians in Medina.
What was Ubaidullah ibn Umar too young to do?
Umar's younger son Ubaidullah killed Hurmuzan, Jafinah, and the daughter of Umar's assassin Abu Lulu. Ubaidullah was intercepted by the people of Medina, who prevented him from continuing the massacre.
What happened as a result of Ubaidullah ibn Umar refusing to follow under his father's orders to not go around killing all the Persians in Medina?
After prolonged negotiations, the tribunal decided to give blood money to the victims, and released Umar's son Ubaidullah

Question: Who stated this?
Self-contained rewrite: Who stated that a tribunal could decide the fate of Umar's younger son Ubaidullah?