Musnad Abu Ya'la al-Mawsili 155:344

Uthman ibn Abi Shaybah narrated to us, Abu Khalid narrated to us, from Yazid ibn Kaysan, from Abu Hazim, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Prompt your dying ones [to say]: 'There is no god but Allah' (La ilaha illa Allah)."