Sahih Bukhari · Hadith 649

قال شعيب وحدثني نافع، عن عبد الله بن عمر، قال تفضلها بسبع وعشرين درجة‏.‏

Narrated `Abdullah bin `Umar: The reward of the congregational prayer is twenty seven times greater (than that of the prayer offered by a person alone).

Gradings are classical jarḥ-wa-taʿdīl, name-matched to a rijāl index (not individually re-verified).