It is likewise the opinion of Ibn Sīrīn, Aḥmad, Isḥāq, and Abū Thawr. A similar view is reported from Ibn Masʿūd, Masrūq, al-Ḥasan al-Baṣrī, Makḥūl, Mujāhid, and Ḥammād ibn Abī Sulaymān: one must not perform the Sunnah of Fajr once the imām has commenced the obligatory prayer.

Mālik said: If he is not afraid that the imām will miss him by a rakʿah, he should pray the Sunnah outside the mosque before entering; but if he fears missing the rakʿah, he should join the imām.

Al-Awzāʿī, Saʿīd ibn ʿAbd al-ʿAzīz, and Abū Ḥanīfah said: Pray the two rakʿahs in a corner of the mosque so long as you are certain you will catch the final rakʿah; if you fear missing that last rakʿah, then join the imām immediately.

Our evidence is the ḥadīth of Abū Hurayrah in which the Prophet (peace and blessings be upon him) said:
“When the iqāmah for the prayer has been called, there is no prayer except the prescribed one.”
Muslim narrated it.

There is also the report of Ibn Buḥaynah: the Messenger of Allah (peace and blessings be upon him) passed by a man after the iqāmah for Fajr had been given and spoke to him about something—though we did not know what it was. When we had finished the prayer, we surrounded the man and asked, “What did the Messenger of Allah say to you?” He replied, “He said, ‘It is only a matter of time before one of you prays Fajr as four rakʿahs!’” Al-Bukhārī and Muslim both related this; the wording above is Muslim’s, while al-Bukhārī’s version reads: “The Messenger of Allah (peace and blessings be upon him) saw a man praying two rakʿahs after the iqāmah had been given. When he finished, he said, ‘Is Fajr four rakʿahs?’”

ʿAbd Allāh ibn Sarjis reported: “A man entered the mosque while the Messenger of Allah (peace and blessings be upon him) was leading the dawn prayer. The man prayed two rakʿahs in a corner of the mosque and then joined the Messenger of Allah. When the Messenger of Allah concluded the prayer he said, ‘So-and-so, which of the two prayers did you count—the prayer you prayed alone or the one you prayed with us?’” Muslim narrated it.

(Branch)
Supererogatory prayers are valid and accepted even when a person’s obligatory prayers are deficient, as established by the two ḥadīths of Abū Hurayrah and Tamīm al-Dārī cited above in the ninth and tenth questions.

As for the ḥadīth transmitted from ʿAlī (may Allah be pleased with him) in which the Prophet (peace and blessings be upon him) is reported to have said, “The likeness of the worshipper is that of a merchant: he realizes no profit until his capital is secured; likewise the worshipper’s supererogatory prayer is not accepted until he performs the obligatory,” it is weak. Al-Bayhaqī and others have made its weakness clear. Al-Bayhaqī added that, were it authentic, it would be understood to refer to a supererogatory prayer whose validity depends on the validity of an obligatory prayer—such as the Sunnahs after Maghrib, ʿIshāʾ, and Ẓuhr—so as to reconcile it with the two ḥadīths of Abū Hurayrah and Tamīm. Allah knows best.