Sunan Abi Dawud Book 5, Hadith 18
The Messenger of Allah (ﷺ) saw a person praying after the congregational prayer at dawn was over. The Messenger of Allah (ﷺ) said: There are two rak'ahs of the dawn prayer (i.e. the prescribed rak'ahs). The man replied: I did not pray the two rak'ahs before the dawn prayer. Hence I offered them now. The Messenger of Allah (ﷺ) kept silent.
The Messenger of Allah (ﷺ)as saying: When the iqamah is pronounced for prayer, no prayer is valid except the obligatory prayer.
'Abd Rabbihi and Yahya b. Sa'id also narrated this tradition from the Prophet (ﷺ) omitting the name of the Companion (mursal). Their grandfather Zaid prayed along with the Prophet (ﷺ).