Sahih Muslim Book 4, Hadith Number 1270.

Chapter : He who came up with the Rak’ah, he in fact came up with the prayer (led in congregation with the Imam).

‘Aisha reported: The Apostle of Allah (may peace be upon him) said the afternoon” prayer as the sun shone in my apartment, and the afternoon shadow did not extend further. Abu Bakr said: The afternoon shadow did not appear to extend further.