Bukhari

Hadith Commentary:
This is an excerpt from a hadith which Isma'il has narrated in full.
In it, it is mentioned that Rifa'ah said "Allahu Akbar" at the beginning of the prayer.
In another narration, it is stated that he said "Allahu Akbar Kabiran."
Imam Bukhari did not narrate the entire hadith because the rest of it is unrelated to this chapter.
Secondly, it is mawquf (stopped at the Companion).

Hadith Commentary:
➊ Imam Bukhari rahimahullah has established from this hadith that Hazrat Rifa‘ah bin Rafi‘ radi Allahu anhu was an Ansari and a participant in the Battle of Badr.

➋ Allamah Isma‘ili has narrated this hadith in such a way that Hazrat Rifa‘ah radi Allahu anhu said "Allahu Akbar" at the beginning of the prayer. In another narration, it is mentioned that he said "Allahu Akbar Kabiran."

➌ Imam Bukhari rahimahullah did not mention the complete hadith because it was mawquf (a narration attributed to a companion) and also because it was beyond the scope of his objective.
(Fath al-Bari: 7/400)

In any case, Hazrat Rifa‘ah radi Allahu anhu was a Badri companion and he passed away at the beginning of the rule of Sayyiduna Amir Mu‘awiyah radi Allahu anhu.
And Allah knows best.