Musnad Abu Ya'la al-Mawsili 160:108

And with his chain of narration, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Shall I not guide you to that by which Allah erases sins and raises degrees?" They said: "Yes, O Messenger of Allah." He said: "Performing ablution thoroughly despite difficulties, increasing the number of steps to the mosques, and waiting for the prayer after the prayer; for that is the ribat (guarding the frontier)."