"We about to die salute thee" said by the Roman gladiators as they marched by the Emperor before fighting.

From Suetonius, "Life of Claudius" (21) : ‘Ave, Imperator, morituri te salutant"
There's a variant Ave, Caesar, ..."

"Hail, Emperor [Caesar], those about to die salute thee"

Actually, the quotation in Suetonius was spoken to Claudius by 'naumacharii' (participants in a mock naval battle, not gladiators) when he was staging a mock naval battle just before he drained the Fucine Lake. When the 'naumacharii' shouted out to him, "Have imperator, morituri te salutamus!" ("Good-bye, Emperor; those about to die wish you well!"), Claudius replied, "Aut non" ("Or not [die]"), after which the 'naumacharii' refused to fight, taking this utterance of Claudius to mean that he had excused them from doing so. But in reality it was a sick joke on Claudius' part. To quote Suetonius:

"Having hesitated for a good while as to whether he should kill them off with fire and sword, at last he leapt forth from his seat, and running, not without unseemly tottering, to and fro around the circuit of the lake, partly by threatening them and partly by cheering them on, forced them to fight." [Divus Claudius 21.6]

The notion that Roman gladiators habitually used this phrase, or some variant of it, to address the Emperor presiding over the games before they engaged in combat is contradicted by the fact that not only Emperors, but public magistrates, provincial governors, and even private individuals sponsored such games (although after 96 CE games in Rome itself could be sponsored only by Emperors, those held outside of Rome requiring official sanction from the central government), and is most probably due to Hollywood or to ill-researched historical fiction.