Tirmidhi

English Commentary:
1:
To recite the words of the two testimonies (shahadatayn) in the call to prayer (adhan) first twice in a low voice, and then again twice in a loud voice, is called tarji‘.

2:
There is a difference of opinion among the Imams regarding whether tarji‘ is Sunnah in the adhan or not.
The correct view is that both forms of adhan—with tarji‘ and without tarji‘—are permissible, and the narrations regarding tarji‘ are found in the two Sahihs (Sahih al-Bukhari and Sahih Muslim), therefore they are the preferred (rajih) view.
And the claim that “the Companions from whom the narrations of tarji‘ have been reported were being taught, because Abu Mahdhurah radi Allahu anhu, to whom the Prophet sallallahu alayhi wa sallam taught this, first recited it in a low voice and then again in a loud voice,” is not correct,
because Abu Mahdhurah continued to give the adhan with tarji‘ in Makkah, and after him, the adhan continued to be given with tarji‘.

Note:
(In the narration of Abu Mahdhurah radi Allahu anhu, the words of the adhan are not mentioned,
but it is mentioned in general terms that the adhan was recited with tarji‘.
Whereas in the hadith reported with the same chain in al-Nasa’i, only seventeen words of the adhan are mentioned,
and there is no explicit mention of tarji‘ in it,
therefore, the version in al-Nasa’i is munkar (rejected),
because in Sahih Muslim,
Sunan al-Nasa’i, and Ibn Majah (708), the words of the adhan are nineteen,
see the author’s next narration (192) and the narrations of al-Nasa’i numbers 306, 631, 632, 633.)