Bukhari

Hadith Commentary:
That is, combining the noon (zuhr) and afternoon (‘asr) prayers in ‘Arafat is the Sunnah of the Prophet sallallahu alayhi wa sallam alone; whose action besides his can be considered Sunnah, and upon whose Sunnah besides his can you act? In some manuscripts, instead of "tattabi‘ūn" it is "yattabi‘ūn,"
meaning: Whose way do you seek besides his? (Wahidi)
The position of the verifying scholars among the Ahl al-Hadith is that in ‘Arafat and in Muzdalifah, one should combine prayers unconditionally, whether a person is a traveler or not, whether he prays with the imam or alone.
Accordingly, ‘Allamah Shawkani rahimahullah states:
"A consensus exists among the people of knowledge that the imam combines the noon and afternoon prayers at ‘Arafat, and likewise, whoever prays with the imam."
That is, there is consensus among the scholars that in ‘Arafat, the imam will combine the noon and afternoon prayers, and all those who pray with the imam must also combine them.
(Nayl al-Awtar)

Hadith Commentary:
(1)
Due to the existence of differing opinions, Imam Bukhari rahimahullah did not make a definitive and certain decision regarding the reason for combining the prayers at Arafah: is it due to travel, or is it due to the rites of Hajj? If it is due to the rites of Hajj, then both the traveler and the non-traveler will combine the prayers. But if it is due to travel, then it is not permissible for the residents of Makkah to combine the prayers, nor can a person who has taken up residence in Makkah combine them. According to our view, shortening (qasr) and combining (jam‘) the prayers is due to the rites of Hajj. Thus, Abdullah ibn Zubayr radi Allahu anhu states that it is from the Sunnah of Hajj that the Imam delivers the sermon after the sun has passed its zenith, and after completing the sermon, he combines and performs the Zuhr and Asr prayers together. (Fath al-Bari: 3/648)

(2)
There is a difference of opinion regarding what a person should do if he is praying alone: should he combine both prayers, or should he perform them separately at their respective times? From the practice of Ibn Umar radi Allahu anhu, it is understood that even an individual (not in congregation) should combine the prayers. In any case, according to our view, in the plain of Arafat and Muzdalifah, the prayers should be combined in all circumstances—whether one is a traveler or a resident, whether praying with the Imam or alone. And Allah knows best.