Abu Hurairah Radi Allahu Anhu narrated that

✦ Abu Hurairah Radi Allahu Anhu narrated that The Prophet Sallallahu Alaihi wasallam said: When the Iqamah has been called for the Salat then there is no Salat except the obligatory.

Sahih Muslim, Book 4, 1532

✦ Ibn Buhaina Radi Allahu Anhu reported: The dawn prayer had commenced when the Prophet Sallallahu Alaihi wasallam saw a person observing prayer, whereas the Mu'adhdhin had pronounced the Iqama. Upon this he (the Prophet Sallallahu Alaihi wasallam ) remarked: Do you say four rak'ahs (of Fard) in the dawn prayer?

Sahih Muslim, Book 4, Hadith 1536

✦ Abdullah bin Sarjis reported A person entered the mosque, while the Messenger of Allah Sallallahu Alaihi wasallam was leading the dawn prayer. He observed two rak'ahs in a corner of the mosque, and then joined the Messenger of Allah Sallallahu Alaihi wasallam in prayer. When the Messenger of Allah Sallallahu Alaihi wasallam had pronounced salutations (he had concluded the prayer), he said: O, so and so, which one out of these two prayers did you count (as your Fard prayer), the one that you observed alone or the prayer that you observed with us?

Sahih Muslim, Book 4, 1537