Bukhari

Hadith Commentary:
In such a manner that, leaving aside the disbelievers, you begin fighting among yourselves.
The apparent meaning of the hadith indicates that shedding the blood of a Muslim without a valid Shar‘i reason is disbelief (kufr).
This is also the statement of Ibn Abbas (radi Allahu anhu), but other scholars have interpreted it differently.
The meaning is that you should not act like the disbelievers.
Abdullah ibn Umar (radi Allahu anhu) was in doubt regarding the farewell—whether the farewell of the Prophet (sallallahu alayhi wa sallam) was intended, or the farewell of Makkah was intended.
However, it later became clear that it was the Prophet’s (sallallahu alayhi wa sallam) own farewell that was intended.
The Prophet (sallallahu alayhi wa sallam) passed away only a few days after this.
This sermon of the Prophet (sallallahu alayhi wa sallam) is also the sermon of the Farewell Pilgrimage (Hajjat al-Wada‘).

Hadith Commentary:
1.
The knowledge of the reason for naming it "Hajjat al-Wada‘" (the Farewell Pilgrimage) was known only to certain distinguished individuals, that by it was meant the sorrowful passing of the Messenger of Allah (sallallahu alayhi wa sallam).
The common people used the term "Hajjat al-Wada‘" but were unaware of its true reason for naming.
The Messenger of Allah (sallallahu alayhi wa sallam) had said that perhaps after this year he would not be able to see them again, so the common people thought that after this year he would not perform Hajj again with such a gathering.
For this reason, they called this Hajj "Hajjat al-Wada‘", but when he passed away a few months later, people realized that the farewell address of the Messenger of Allah (sallallahu alayhi wa sallam) was not restricted to a particular group, but rather he was bidding farewell to both the elite and the general public.
2.
It is narrated in Sahih Muslim that the Messenger of Allah (sallallahu alayhi wa sallam) said:
"Perhaps after this Hajj I will not be able to perform Hajj again, so learn from me the rites of Hajj."
(Sahih Muslim, al-Hajj, Hadith: 3137(1297))
Similarly, from other narrations, it is understood that the Messenger of Allah (sallallahu alayhi wa sallam) was near to his passing, and for this reason it was called Hajjat al-Wada‘.
In any case, most of the noble Companions (radi Allahu anhum) were not aware that the Messenger of Allah (sallallahu alayhi wa sallam) was departing from them, or that someone else was bidding them farewell.
When he passed away, it became clear that he had given people his bequest and bade them farewell, and he had advised them not to act like the disbelievers after him, then with Allah as witness, he further strengthened this farewell.
(Fath al-Bari: 8/133)