When a latecomer wants to join the congregational Salah
a person, who misses part of the Salah, enters the Masjid (mosque) and wants to catch up with the Imam, he should walk in tranquility and dignity until he reaches the row. He should then join the row and say Takbirat-ul-Ihram (saying: "Allahu Akbar [Allah is the Greatest]" upon starting Prayer).
Muslim narrated on the authority of Abu Hurayrah (may Allah be pleased with him) who said, I heard the Prophet (peace be upon him) saying,
When the Iqamah (call to start the Prayer) has been pronounced for prayer, do not go running to it, but go walking in tranquility and pray what you catch up with, and complete what you have missed.
Abu Bakrah once joined the Prophet (peace be upon him) while in the bowing position, so he bowed before reaching the row to catch Ruku'. After the Salah was over, he mentioned this act to the Prophet (peace be upon him) who said,
May Allah increase your love for good but do not repeat it again (bowing in that way.)
Al-Bukhari) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.