Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "Remain upright, though you will never be able to do it fully, and know that the most excellent of your deeds is prayer, and none but a believer will guard the prayer regularly."
Al-Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains al-Waqidi, who is da'if (weak).