In the Inheritance Cycle, when
he tells Eragon seven words in the ancient language, adjuring him only to use them in a time of great need.
“It is the way of things . . . I must. Will you take my blessing?” Eragon bowed his head and nodded, overcome. Brom placed a trembling hand on his brow. “Then I give it to you. May the coming years bring you great happiness.” He motioned for Eragon to bend closer. Very quietly, he whispered seven words from the ancient language, then even more softly told him what they meant. “That is all I can give you.... Use them only in great need.”
Now, Eragon obtains significantly better instruction in magic later on, compared to what he received from Brom. So he might already have learned these words. He also has had what could be considered many "times of great need," with no mention of the words.
Nonetheless, is there any in-story evidence or authorial confirmation as to what those words might be?