Abu Barzah (may Allah be pleased with him) says that the Prophet (peace and blessings be upon him) said to me: "If you are able, stand behind the imam; otherwise on the right side of the row." He said: This is how Abu Bakr and Umar (may Allah be pleased with them both) used to be behind the Prophet (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5202