Musnad Abu Ya'la al-Mawsili 42:144

Ishaq narrated to us, Bishr bin al-Fadl narrated to us, from 'Umarah bin Ghaziyyah, from Yahya bin 'Umarah, he said: I heard Abu Sa'id al-Khudri say: The Messenger of Allah (may Allah bless him and grant him peace) said: "Prompt your dying ones [to say]: 'There is no god but Allah' (La ilaha illa Allah)."