Nasai

866. Commentary: When the call to establish (iqamah) any obligatory (fard) prayer is given, it is not permissible to begin any supererogatory (nafl) or other obligatory prayer, because this is contrary to the principles of congregation (jama‘ah), and it would diminish the importance of the congregational prayer. However, if a person is already performing sunnah or similar prayers and there is no fear that by continuing he will miss any part of the obligatory prayer (such as if he is in the tashahhud), then according to one scholarly opinion, he may continue the prayer and try to complete it quickly so that he can join the congregational obligatory prayer. If he fears that by continuing, he will miss some part of the congregational obligatory prayer or miss a rak‘ah, then he should break off his prayer and join the congregation. However, it is better that as soon as the iqamah begins, he should abandon his prayer, regardless of which stage of the prayer he is in, because from the clear text of «فَلَا صَلَاةَ», it is understood that such a prayer is not counted, even if he, in his own estimation, continues the prayer.

Commentary:
1:
That is, when the iqamah begins.

2:
That is, for whom the iqamah is being called.

3:
This hadith indicates that once the takbir (opening Allahu Akbar) for the obligatory prayer has been pronounced, it is not permissible to perform voluntary (nafl) prayers, even if they are the regular sunnah (rawatib). Some people have made an exception for the sunnah of Fajr, but this exception is not correct, because in the narration of Muslim bin Khalid, which he narrated from Amr bin Dinar, it is further mentioned:
((It was said: O Messenger of Allah, not even the two rak‘ahs of Fajr? He said: Not even the two rak‘ahs of Fajr.))
Ibn ‘Adi has recorded this in al-Kamil under the biography of Yahya bin Nasr bin Hajib, and has called its chain hasan (good).
As for the narration of Abu Hurayrah, which al-Bayhaqi has recorded and in which there is the addition ((except the two rak‘ahs of Fajr)), the answer is that regarding this addition, al-Bayhaqi himself says: "This addition has no basis," meaning this addition is baseless. In its chain are Hajjaj bin Nasr and ‘Abbad bin Kathir, and both are weak. Therefore, it is not correct to use this as evidence.

Prayer After the Iqamah Has Been Called
The Noble Prophet sallallahu alayhi wa sallam said: When the iqamah for prayer has been called, there is no other prayer except the obligatory one. [صحيح مسلم ج1 ص247 ح710]

In opposition to this authentic hadith, it is written:

“It is permissible to perform the Sunnah prayers of Fajr even when the congregation has stood for prayer.” [چهل حديث ص104 !]

In a narration attributed to Abu Uthman al-Hindi, it is mentioned that we used to come to Umar (radi Allahu anhu) before performing the two rak‘ahs before Fajr, while he would be leading the prayer. We would perform the two rak‘ahs of Sunnah at the back of the mosque and then join the congregation with the people. [شرح معاني الآثار 376/1، آثار السنن: 727]

Its chain is weak due to Ja‘far ibn Maymun («ضعيف عند الجمهور»).

See my book: Anwar al-Sunan fi Tahqiq Athar al-Sunan [مخطوط ص146]

Regarding Ja‘far ibn Maymun, Imam Ahmad ibn Hanbal rahimahullah said:
He is not strong in hadith. [كتاب العلل و معرفة الرجال ص58 فقره: 4157]

Therefore, it is incorrect for Nimawi to call this chain hasan.

Sayyiduna Ibn Umar radi Allahu anhuma saw a man performing two rak‘ahs at the time of iqamah, so he struck him with pebbles and said: Are you performing four rak‘ahs? [السنن الكبري للبيهقي 483/2 و سنده صحيح]

It is established from an authentic hadith that whoever misses the two Sunnah rak‘ahs of Fajr and then performs them immediately after the obligatory prayer, it is permissible.
See: [صحيح ابن خزيمه ج2 ص164 ح1116

صحيح ابن حبان الاحسان: 2462 اور المستدرك للحاكم 274/1۔ 275 ح1017]

Both al-Hakim and al-Dhahabi have also declared this narration authentic, and the criticism of Ibn ‘Abd al-Barr on this narration is rejected.

In one narration, it is mentioned that whoever has not performed the two rak‘ahs of Fajr should perform them after the sun has risen.
[سنن الترمذي: 423، المستدرك 307/1 ح1153]

Its chain is weak and rejected due to the tadlis (an‘anah) of Qatadah the mudallis (see above: 31).

... See the original article ...

Fatawa ‘Ilmiyyah (Tawdih al-Ahkam) vol. 2, p. 77

1266. Commentary:
From this hadith as well, the prohibition of performing Sunnah prayers while the congregational (jama‘ah) prayer is established is proven. And the narration of al-Bayhaqi which states, "When the congregational prayer is established, there is no prayer except the obligatory (fard) prayer, except for the Sunnahs of Fajr," is completely baseless and weak. See: [عون المعبود]

Benefit:
When the congregational prayer (jama‘ah) stands, one should join it.
At that time, it is not permissible to perform any sunnah or voluntary (nafl) prayers.
Therefore, if a person is performing sunnah prayers and the congregational prayer begins, he should leave the sunnah and join the congregation.
This is the preferred (rajih) and closest to correctness (aqrab ila al-sawab) opinion.
However, some scholars say that if he is certain that the sunnah or voluntary prayers he is performing will be completed before the opening takbir (takbir tahrimah), then he may complete them; otherwise, he should not.
And Allah knows best.