Sayyiduna Ibn Buhaina (may Allah be pleased with him) said: The iqamah for prayer had been given when the Messenger of Allah (peace and blessings be upon him) noticed a man who was performing two units of Sunnah prayer. When the Prophet (peace and blessings be upon him) finished the prayer, the people surrounded that man. The Messenger of Allah (peace and blessings be upon him) said: "Are you performing four units of (Fajr) morning prayer?"
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1488
That is, after the iqamah (call to commence prayer), performing any prayer other than the obligatory prayer was astonishing and unknown. That is why the noble Companions surrounded him and sat down, and the Prophet sallallahu alayhi wa sallam, expressing disapproval, said: "Do you perform the morning prayer as four rak‘ahs?"