Sayyiduna Anas (may Allah be pleased with him) narrates this saying of the Prophet (peace and blessings be upon him): "Prayer in congregation is superior to praying alone (the narrator is unsure; perhaps the words are:) to a man's praying by himself, twenty-five times over."
Imam al-Bazzar and Imam al-Tabarani in al-Mu'jam al-Awsat recorded it; the narrators of Imam al-Bazzar are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2135