Musnad Ahmad 31:9

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