Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1252
Abu Huraira reported: The Messenger of Allah (may peace be upon him) said: When the words of Iqama are pronounced, none of you should run to it (to join the prayer) but walk with tranquillity and dignity, and pray what you are in time for and complete what has gone before (what the Imam has completed).