Al-Mughirah bin Shu'bah (may Allah be pleased with him) says: The Messenger of Allah (peace and blessings be upon him) and I arrived when Abd al-Rahman bin Awf (may Allah be pleased with him) had already led the people in one rak'ah. He (may Allah be pleased with him) began to step back, but the Messenger of Allah (peace and blessings be upon him) gestured to him to remain in his place. We prayed the part of the prayer (in congregation) that we caught, and afterwards completed what we had missed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3887