Tirmidhi

English Commentary:
1:
The meaning of separating by means of salam is that he used to perform the four rak‘ahs by offering them as two sets of two rak‘ahs each. The believers are called followers of the close angels because, like the angels, the believers also have faith in the oneness (tawhid) of Allah and His greatness.

Note:
(In the chain of narration, Abu Ishaq al-Sabi‘i is a mudallis, and the narration is with ‘an‘anah, but based on corroborating evidence, this hadith is authentic.)

875. Commentary:

➊ The first voluntary prayer refers to Salat al-Duha (the forenoon prayer). If this prayer is performed when the sun has risen to the height of a spear or two spears, it is called Salat al-Ishraq. In any case, Salat al-Ishraq, Salat al-Duha, and Salat al-Awwabeen are different names for the same prayer. These names differ only due to the change in timing. And Allah knows best. For details, see: [القول المقبول ، ص : 688]
And the second voluntary prayer refers to the Sunnah of Zawal, because it is performed before the sun begins to decline.

➋ The greeting (salam) here refers to reciting during the Tashahhud: «السَّلامُ علَينا و على عبادِ اللهِ الصّالحينَ» “Peace be upon us and upon the righteous servants of Allah,” not the concluding salam at the end of the prayer. And the mention of angels, prophets, and others is an explanation of the term “the righteous.”