Musnad Ahmad 30:902

I read to Abd al-Rahman: Malik, Ishaq narrated to us, he said: Malik narrated to us, from al-Ala’ ibn Abd al-Rahman, from his father, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Shall I not inform you of that by which Allah erases sins and raises degrees?" "Performing wudu thoroughly despite difficulties" — Ishaq said: "in difficulties" — "and the abundance of steps to the mosques, and waiting for the prayer after the prayer; for that is the ribat, for that is the ribat, for that is the ribat."