The Messenger of Allah ﷺ saw a man performing the two rakahs of Fajr just as the muadhin began the iqamah. The Prophet ﷺ tapped his shoulder and said, "Should this not have come before that?"
أن رسول الله ﷺ رأى رجلا يصلي ركعتي الفجر حين أخذ المؤذن يقيم، فغمز النبي الله ﷺ منكبه وقال: ألا كان هذا قبل هذا؟
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Tabarani narrated it in Al-Kabir and Al-Awsat, as found in Majma Bahrain (771), from Ahmad bin Hamdan Abu Saeed Tustari at Abadan, who said, Ibrahim bin Yusuf Sayrafi told us, who said, Abdul-Rahman bin Muhammad Muharibi told us, from Sulayman Shaybani, from Abu Bakr bin Abu Musa, from his father, and he related it.Its chain is hasan because of Ibrahim bin Yusuf Sayrafi, for there is disagreement about him, though his hadith is hasan. Haythami said in Majma al-Zawaid (752), "His narrators are trustworthy." Shawkani quoted in Nil (2/314) from Iraqi that he said, "Its chain is good."On this subject there are also narrations from Zayd bin Thabit, Ayesha, and Ibn Umar, though there is discussion regarding their chains. The hadiths on this subject indicate that it is disliked to perform the sunnah prayer of Fajr when the call to begin the obligatory prayer has been given.As for what Ibn Majah (1147) narrated from the hadith of Ali , that he said, "The Prophet ﷺ used to pray the two units of prayer when the call to begin the prayer was given," its chain contains Harith Awar, who is daif, and indeed he was accused of lying. Busiri weakened it in Zawaid bin Majah. Moreover, it contradicts what has been established from the Messenger of Allah ﷺ, that he used to hasten to perform them upon hearing the call to the Fajr prayer.