Musnad Abu Ya'la al-Mawsili 3:228

Ahmad ibn Ibrahim al-Dawraqi narrated to us, Safwan ibn Isa al-Zuhri narrated to us, al-Harith ibn Abd al-Rahman narrated to us, from Sa'id ibn al-Musayyib, from Ali ibn Abi Talib, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Performing wudu thoroughly despite difficulties, walking frequently to the mosques, and waiting for the prayer after the prayer washes away sins completely."