- 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)
The Doctor used the two times, one for riding a horse out of the TARDIS with Queen Elizabeth I and using his TARDIS with Eleventh Doctor, War Doctor and other Doctors to save Gallifrey. (TV: The Day of the Doctor)