Authentic Supplication Number 103.

The Prophet  said to a man: “What do you say in your salat?”, He said: I say the tashahhud and then I say: ‘Allahumma I ask You Paradise and I seek refuge in You from Hell-Fire. I am not well capable of the like of your murmuring, nor the murmuring of Mu’ath, the Prophet  then said: “It is about that that we murmur”.

Found in Sunan Abi Dawood. Al-Albani said it was of sound ascription, and that An-Nawawi and Al-Bouseeri said it was sound. It is # 86 in The Authentic of Good Sayings.