Sayyiduna Abu Burdah al-Aslami (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) said: "If you are able, stand behind the imam; otherwise, stand on his right."
Al-Tabarani recorded it in al-Mu'jam al-Awsat. Its chain contains a narrator of whom I have found no mention.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2514