(a) Ubaydullah bin Abdullah bin Utbah narrates from his father: I came to Umar bin al-Khattab (may Allah be pleased with him) at midday. I found him offering voluntary prayer, and I stood behind him, so he took hold of me and placed me on his right side. When he came, I moved back in accordance with him, and we formed a row behind Umar (may Allah be pleased with him).
(b) It is reported from Umar and Ali (may Allah be pleased with them both) that when there were three, two would stand behind (the imam).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5159