Musnad Abu Ya'la al-Mawsili 115:92

Hureym ibn ‘Abd al-A‘la Abu Hamzah al-Asadi narrated to us, Mu‘tamir narrated to us, he said: I heard my father narrating from Qatadah, from Anas, he said: The general part of the final instruction of the Messenger of Allah (may Allah bless him and grant him peace) when death approached him was: "The prayer, and what your right hands possess." He continued to gargle it, or gargle with it, in his chest, while his tongue could not utter it clearly.