It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Shall I not tell you that by which Allah Almighty wipes away sins and raises ranks? Completing the ablution despite hardship (i.e., severe cold), walking often to the mosques, and waiting for one prayer after another; this is ribat." He said it three times. (Ribat means guarding the frontiers.)
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 386