Musnad Abu Ya'la al-Mawsili 155:307

Suwayd ibn Sa'id narrated to us, Dammam narrated to us, from Musa ibn Wardan, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Increase in the testimony that there is no god but Allah before it is prevented between you and it."