Allons-y

You may be looking for the Doctor Who Confidential episode, Allons-y!.

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 Tenth Doctor yelled the phrase as he, along with twelve other incarnations, combined the power of their TARDISes to use a stasis cube to freeze Gallifrey in a pocket universe. (TV: The Day of the Doctor)

The first time the Doctor used the catchphrase was in the Tardis, using it instead of Let's go. He then takes a liking to it and says 'Allons-y, I should say that more often.' (TV: Army of Ghosts (TV story)

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)

Mr Clever once used it when mimicking the Tenth Doctor through the Eleventh Doctor's body. (TV: Nightmare in Silver)

The Tenth Doctor is also the Only Modern Doctor to take a while to get his Catchphrase, as the Ninth Doctor used his in Rose (TV story) and The Eleventh used his straight after the Tenth Doctor's Regeneration End of time