"Dragons rise! You cannot stop us!"
At one point in Rise of Tiamat, the party fights some cultists, and the adventure states:
With their last breaths, dying cultists should curse the characters and exclaim, “The dragons rise! You cannot stop us!”
During another encounter, we see:
Don’t be afraid to engage in some triumphal monologuing by the dragonsoul cultist leading the attack. Whether the attackers are winning or losing, the cultist fights with cries of “The dragons rise!” and “We are unstoppable!”
These utterances are probably the closest to what you are after, but there are some other sayings that may be eligible candidates for what you are after.
Cult Signs and Signals
In Hoard of the Dragon Queen, we are given some information about common pass phrases used by the cult:
In addition, the phrases “All hail Tiamat!” and “They shall rise!” are frequently used pass phrases.
Elsewhere we learn another pass phrase:
Tiamat, our Mother and Strength
We also find a magical spear belonging to the cult that can be activated with the command phrase:
Tiamat’s eyes shine
These may also serve as exclamations a cultist might use.
"Praise Tiamat's glory"
During HotDQ, the party encounters a camp of cultists, and can learn some information:
Characters can learn the following information through observation and questioning.
In particular, there are 12 bullet points in this section that the characters may learn about the cult, and three of them contain the exclamation:
praise Tiamat’s glory!
The idea here seems to be that as cultists divulge information about the cult and about Tiamat and her ambitions, they frequently utter "praise Tiamat's glory!" as an expression of their piety.