(a) Abd al-Rahman bin al-Aswad (may Allah have mercy on him) narrates from his father: Alqamah (may Allah have mercy on him) and I came to Abdullah bin Mas'ud (may Allah be pleased with him) at midday. When the sun had passed its zenith, he established the prayer, and my companion and I stood behind him. He took my hand and my companion's hand and made us stand on his right and left, and he himself stood between us, and said: "This is what the Messenger of Allah (peace and blessings be upon him) used to do when there were three people." Then he led us in prayer. When he had finished, he said: "Soon there will be imams who delay the prayers; do not wait for them, and pray with them as well; it will be a voluntary prayer for you."
(b) Abu Dharr (may Allah be pleased with him) says that Ibn Mas'ud (may Allah be pleased with him) saw him (peace and blessings be upon him) and his Companions (may Allah be pleased with them) praying voluntary prayer separately; he did not lead them in congregation.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5171