Musnad Abu Ya'la al-Mawsili 42:123

Al-Abbas ibn al-Walid al-Narsi narrated to us, Bishr ibn al-Mufaddal narrated to us, Umarah ibn Ghaziyyah narrated to us, from Yahya ibn Umarah, who said: I heard Abu Sa’id al-Khudri say: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Prompt your dying ones [to say]: 'La ilaha illa Allah' (There is no god but Allah)."