On it being disliked to start doing voluntary prayers after the iqama has begun, even if it is the sunna for that prayer

Riyad as-Salihin (The Meadows of the Righteous)

by Imam Nawawi

344. Chapter: On it being disliked to start doing voluntary prayers after the iqama has begun, even if it is the sunna for that prayer

1759. Abu Hurayra reported that the Prophet, may Allah bless him and grant him peace, said, "When the iqama for the prayer is given, there is no prayer but the obligatory prayer." [Muslim]