(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
Sayyiduna Abu Dharr (may Allah be pleased with him) says that one night the Messenger of Allah (peace and blessings be upon him) stood at such-and-such a place and prayed the Isha prayer there. When he (peace and blessings be upon him) saw that the people had stayed with him at his place of prayer, he (peace and blessings be upon him) went off to his lodging, until, when eyes had grown heavy with sleep and the place where he (peace and blessings be upon him) had stood was empty, he (peace and blessings be upon him) stood there alone. Sayyiduna Abu Dharr (may Allah be pleased with him) says: I came and stood behind him (peace and blessings be upon him). He (peace and blessings be upon him) signalled to his right, and Sayyiduna Ibn Mas'ud (may Allah be pleased with him) came and stood behind him (peace and blessings be upon him) and behind me; he (peace and blessings be upon him) signalled him to his left, and we stood like this. He joined together his index finger, his middle finger and the finger next to the little finger. Each of us was praying his own prayer.
Note: Al-Humaydi (may Allah have mercy on him) says that Sayyiduna Ibn Mas'ud (may Allah be pleased with him) understood that the Prophet (peace and blessings be upon him) had led them as imam, but Sayyiduna Abu Dharr (may Allah be pleased with him) made clear that each one prayed his own prayer; the Prophet (peace and blessings be upon him) did not lead them as imam.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5172
Hisham bin Hassan (may Allah have mercy on him) says: I mentioned (this) before Ibn Sirin (may Allah have mercy on him), and he said: Ibn Mas'ud (may Allah be pleased with him) did that because the mosque was narrow.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5173