Musannaf Ibn Abi Shayba 1:44

Yahya ibn Abi Bukayr narrated to us, he said: Zuhayr ibn Muhammad narrated to us, from Abdullah ibn Muhammad ibn Aqil, from Sa'id ibn al-Musayyib, from Abu Sa'id, that he heard the Messenger of Allah (may Allah bless him and grant him peace) saying: "Shall I not guide you to something by which Allah effaces sins and increases good deeds?" They said: "Yes, O Messenger of Allah." He said: "Performing wudu thoroughly despite difficulties, and the abundance of steps toward these mosques."