Allons-y

You may be looking for the Doctor Who Confidential episode.

Allons-y, according to the Tenth Doctor, was French for "Let's go". (TV: Midnight) It was one of the Tenth Doctor's favourite sayings; he especially wanted to say, "Allons-y, Alonso", (TV: Army of Ghosts) which he eventually said to Alonso Frame. (TV: Voyage of the Damned)

It saved his life on Midnight by proving that the being possessing Sky Silvestry had stolen his words. (TV: Midnight)

The last time the Doctor used the phrase, he described it to Addams as a word of consolation to the soul in times of need. (TV: The End of Time)