Narrated Malik Ibn Buhaina:
Allah’s Apostle passed by a man praying two Rakat after the Iqama (had been pronounced). When Allah’s Apostle completed the prayer, the people gathered around him (the Prophet) or that man and Allah’s Apostle said to him (protesting), Are there four Rakat in Fajr prayer? Are there four Rakat in Fajr prayer?”
Prophet (SAW) was harsh about it because he wanted to make this clear: no Salah is to be offered except the compulsory Salah after the Iqaamah has been pronounced for that compulsory Salah.
Simply, if you’re praying in the masjid/with Jama’ah, you can’t pray any Salah in between the Iqaamah and the Jama’ah (congregation).
Remember, Adhaan and Iqaamah are different.