Abu Hurairah (May Allah be pleased with him) reported: I heard the Messenger of Allah (PBUH) saying, "When the Iqamah are pronounced, do not come to it running, you should walk calmly with tranquillity to join the congregation. Then join in what you catch for and complete what you miss."[Al-Bukhari and Muslim].
In Muslim it is added: Messenger of Allah (PBUH) said, "For when one of you is walking for Salat, he is, in fact, engaged in Salat."
This Hadith prevents us from running or walking hurriedly in order to join a congregational prayer because this is undignified. Whereas, we are commanded to be self-composed and dignified with regard to all matters. Secondly, when a believer walks up to the mosque after performing ablution at home, he is considered to be in a state of Salat. Thirdly, the first Rak`ah he prays behind the Imam will be counted his first Rak`ah, so he must make up for the Rak`ah he may have missed, if any, after the Imam has concluded his Salat.