Musnad Abu Ya'la al-Mawsili 42:266

Zuhayr narrated to us, Abu 'Amir al-'Aqdi narrated to us, from Sulayman ibn Bilal, from 'Umarah ibn Ghaziyyah, from Yahya ibn 'Umarah, from Abu Sa'id, that the Prophet (peace be upon him) said: "Prompt your dying ones [to say]: 'There is no god but Allah' (La ilaha illa Allah)."