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 use 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 (TV story).