Bukhari

Hadith Commentary:

That is, Abdullah ibn Mas'ud (radi Allahu anhu) was just saying this when Uthman (radi Allahu anhu) returned from Muzdalifah. The Sunnah is that one should return from Muzdalifah after the break of dawn (fajr) but before the sun rises.

Regarding the time of the Fajr prayer mentioned in this hadith, where it is stated that it was performed at such a time that people were in doubt as to whether Fajr had actually begun, its clarification is found in the hadith of Sahih Muslim narrated from Jabir (radi Allahu anhu): The Prophet (sallallahu alayhi wa sallam) combined Maghrib and Isha, then he slept. "Then the Messenger of Allah (sallallahu alayhi wa sallam) lay down until the dawn broke, and he performed the Fajr prayer when the morning became clear to him, with adhan and iqamah," until the end of the hadith. Then, after sleeping, he (sallallahu alayhi wa sallam) stood up when Fajr had appeared.

He (sallallahu alayhi wa sallam) performed the Fajr prayer after the morning had become clear, and for it, both adhan and iqamah were called.

It is thus understood that in the previous hadith, the narrator means that he (sallallahu alayhi wa sallam) performed the Fajr prayer at the very earliest time, that is, immediately upon the appearance of Fajr, while it was still dark. In this way, the Prophet (sallallahu alayhi wa sallam) would always perform the Fajr prayer in darkness (ghalas), as is established by numerous ahadith. However, here, even more so, he (sallallahu alayhi wa sallam) performed the Fajr prayer immediately after the break of dawn.

Hadith Commentary:
(1)
Imam Bukhari rahimahullah has presented two narrations from Abdullah ibn Mas'ud radi Allahu anhu: one concise and one detailed. The purpose of these is to establish two rulings:
➊ The morning prayer (Salat al-Fajr) on the tenth of Dhul-Hijjah should be performed in Muzdalifah. However, those who are weak and unable, both men and women, are permitted to proceed to Mina in the latter part of the night, perform the Fajr prayer in Mina, and then carry out the ritual of stoning (rami) and other rites.
➋ The Fajr prayer should be performed at its proper time, though it may be offered slightly earlier than the usual time. This does not mean that it should be performed before true dawn (subh sadiq), as some have assumed; rather, it must be performed after the break of dawn, as is established in Sahih Bukhari itself, where it is mentioned that the Messenger of Allah sallallahu alayhi wa sallam performed the Fajr prayer at the time of dawn.
(Hadith: 1683)
There is further clarification in Sahih Muslim that the Messenger of Allah sallallahu alayhi wa sallam, after performing Maghrib and Isha, slept until dawn broke, and then, after the morning became clear, performed the Fajr prayer.
In general, his practice was to perform the Fajr prayer in darkness, but on that day, he performed the Fajr prayer immediately after the break of dawn so that there would be some time left for the stay (wuquf) at Muzdalifah.

(2)
Regarding the words Abdullah ibn Mas'ud radi Allahu anhu used about Uthman radi Allahu anhu, Hafiz Ibn Hajar rahimahullah writes that these very words were uttered when returning from the standing at Arafah. Thus, when they performed the standing at Arafah and the sun had set, he said:
If the Commander of the Faithful (Amir al-Mu'minin) were to return to Muzdalifah at this time, he would be acting in accordance with the Sunnah.
The narrator says that I do not know whether Ibn Mas'ud radi Allahu anhu said this first, or whether Uthman radi Allahu anhu set out for Muzdalifah first.
(Fath al-Bari: 3/671)