Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrated that the Messenger of Allah (peace and blessings be upon him) said: "The first call (adhan) was only instituted so that those who pray may get ready for their prayer. So when you hear the adhan, perform ablution (wudu) thoroughly, and when you hear the iqamah, hasten to the opening takbir, for it is an essential part of the prayer and its completion. And do not race ahead of the reciter in bowing and prostration."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12330