'And Ulmo came to [Turgon] once again, and spoke with him. And he said: “Now thou shalt go at last to Gondolin, Turgon; and I will maintain my power in the Vale of Sirion, and in all the waters therein, so that none shall mark thy going, nor shall any find there the hidden entrance against thy will. Longest of all the realms of the Eldalië shall Gondolin stand against Melkor. But love not too well the work of thy hands and the devices of thy heart; and remember that the true hope of the Noldor lieth in the West and cometh from the Sea.”' - Of the Noldor in Beleriand, The Silmarillion.
Ulmo and Turgon, by Alexey Buteev