Bukhari

Hadith Commentary:

Usamah bin Zayd radi Allahu anhuma was born from Umm Ayman, who was the freed slave woman of your (sallallahu alayhi wa sallam) father, Abdullah, and who had raised the Prophet sallallahu alayhi wa sallam.

Usamah was the most beloved servant of the Prophet sallallahu alayhi wa sallam.

At the time of the Prophet’s (sallallahu alayhi wa sallam) passing, his age was twenty years.

He was martyred in Wadi al-Qura in the year 54 AH.

Radi Allahu anhu.

Hadith Commentary:
➊
Hazrat Usamah radi Allahu anhu also adopted seclusion during the days of turmoil and discord (fitnah and fasad).
Most likely, the reason for this caution was that during the blessed era of the Messenger of Allah sallallahu alayhi wa sallam, he had killed a person who had professed the testimony of faith (kalimah), and the Messenger of Allah sallallahu alayhi wa sallam expressed intense anger and severe displeasure at this.
From that time onwards, he made it obligatory upon himself that he would never again fight or engage in combat with any Muslim.
In the aforementioned hadith as well, the aspect of Hazrat Usamah radi Allahu anhu’s caution is clearly evident.

➋
The commentator of Sahih Bukhari, Ibn Battal, says that Hazrat Usamah radi Allahu anhu sent his freed slave Harmalah to Hazrat Ali radi Allahu anhu to apologize, saying that in the battles of Siffin and Jamal, he was unable to support him solely out of respect for the sanctity of Muslim blood.
If you had been in any kind of personal distress, you would have found me at your side before anyone else, but the matter of fighting and combat was such that I was unable to participate in them.
And Allah knows best.
(Fath al-Bari: 13/85)