the iqāmah has been pronounced, and a detailed discussion of this point will follow in its proper place, God willing.
I have also uncovered a third line of transmission for this ḥadith. Al-Khaṭīb cites it through Aḥmad ibn Bashshār al-Ṣayrafī, who said: Ḥaddathanā Abū Ḥafṣ al-ʿAbdī; from Ayyūb; from Abū Qilābah; from Sulaymān ibn Bishār; from Abū Hurayrah, in a marfūʿ report with the same wording.
This Aḥmad narrated from a number of teachers and a group of scholars narrated from him. Al-Khaṭīb gives him a brief entry and quotes this very ḥadith, yet records neither criticism nor commendation.
As for Abū Ḥafṣ al-ʿAbdī, I am unfamiliar with him unless he be ʿUmar ibn Ḥafṣ al-ʿAbdī—or simply Ḥafṣ—who belongs to an earlier generation and narrates from Thābit. If he is indeed that narrator, then he is weak (wāhin), as al-Dhahabī observes.
I have found no information on Sulaymān ibn Bashār. Among narrators of this period, however, there is a Sulaymān ibn Bishr, counted by Aḥmad in al-Taʿjīl and deemed trustworthy by Ibn Ḥibbān.
The ḥadith thus indicates that it is impermissible to begin a voluntary prayer once the iqāmah has been heard, whether that be the Fajr sunnah or any other supererogatory prayer. This is the view of the majority of scholars. Al-Tirmidhī states:
“Some scholars from among the Companions of the Prophet ﷺ and others act on this: when the iqāmah has been called, a man should perform nothing but the obligatory prayer. This is also the opinion of Sufyān al-Thawrī, Ibn al-Mubārak, al-Shāfiʿī, Aḥmad, and Isḥāq.”
A fuller treatment of this issue, God willing, will appear in the section mentioned earlier.