سنن أبي داود

Narrated AbuSa'id al-Khudri:

Prayer in congregation is equivalent to twenty-five prayers (offered alone). If he prays in a jungle, and performs its bowing and prostrations perfectly, it becomes equivalent to fifty prayers (in respect of reward).

Abu Dawud said: 'Abd al-Walid b. Ziyad narrated in his version of this tradition: "Prayer said by a single person in a jungle is more excellent by multiplied degrees than prayer said in congregation."