Nafi' narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Prayer in congregation is superior to the prayer of one alone by twenty-seven degrees."
In the narration of Imam al-Shafi'i it is: "(It surpasses) the prayer of one alone."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4955