'speak friend and enter'
it's the answer to the riddle on the gates of moria which gandalf answers
On 13 January 3019 the Fellowship of the Ring entered Moria through the Doors, but initially Gandalf could not find out the password to open them. Merry Brandybuck unknowingly gave Gandalf the answer by asking, "What does it mean by speak, friend, and enter?" When Gandalf realized that the correct translation was "Say friend and enter" he sprang up, laughed, and said "Mellon", which means "friend" in Sindarin, and the Doors opened. Shortly thereafter, the Watcher in the Water attacked the Fellowship and shut the Doors behind them. Afterwards Khazad-dûm was resettled by Dwarves; it is unknown if the Doors of Durin were ever repaired.