It is narrated from Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Remain upright, though you will never be able to count all your deeds; and know that the best of your deeds is prayer, and none keeps up the prayer except a believer."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6243