Musnad Ahmad 30:2516

Yahya ibn Sa'id narrated to us, from Shu'bah, who said: Al-'Ala' narrated to me, from his father, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him: "Shall I not guide you to that by which Allah raises the ranks and expiates the sins?" [They said:] "Yes, O Messenger of Allah." He said: "Abundance of steps to the mosques, waiting for the prayer after the prayer, and performing wudu' thoroughly despite difficulties."