Musnad Ahmad 207:1

Mu'awiyah ibn 'Amr narrated to us, he said: Za'idah narrated to us, from al-A'mash, from Abu Salih, from some of the companions of the Prophet, may God bless him and grant him peace, who said: The Prophet, may God bless him and grant him peace, said to a man: "How do you speak in the prayer?" He said: "I recite the tashahhud, then I say: 'O Allah, I ask You for Paradise, and I seek refuge in You from the Fire.' As for me, I do not know how to murmur your murmuring, nor the murmuring of Mu'adh." The Prophet, may God bless him and grant him peace, said: "Around it we murmur."