Musnad Ahmad 30:611

'Abd al-Razzaq narrated to us, Malik narrated to us, from al-'Ala' ibn 'Abd al-Rahman, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Shall I not guide you to that by which Allah effaces sins and raises degrees?" They said: "Yes, O Messenger of Allah." He said: "Walking to the mosques, performing wudu' thoroughly despite difficulties, and waiting for the prayer after the prayer; for that is the ribat (the stationing at the frontier)."