Musnad Ahmad 1156:13

Muhammad ibn Abi ‘Adi narrated to us, from Sa‘id, from Qatadah, from Safinah, the freed slave of Umm Salamah, from Umm Salamah, who said: Among the last of the instructions of the Messenger of Allah (may Allah bless him and grant him peace) was: "The prayer, the prayer, and what your right hands possess." The Prophet of Allah (may Allah bless him and grant him peace) continued to repeat it in his chest, and his tongue could not articulate it clearly.