"Such thoughts as these were crowding upon his mind as he stood under the portico of the holy temple. Suddenly the gate and the inner veil of the temple seemed to be uplifted or withdrawn, and he was permitted to gaze within, upon the holy of holies, where even the prophet's feet might not enter. There rose up before him a vision of Jehovah sitting upon a throne high and lifted up, while His train filled the temple. On each side the throne hovered the seraphim, two wings bearing them up, two veiling their faces in adoration, and two covering their feet. These angel ministers lifted up their voices in solemn invocation, “Holy, holy, holy, is the Lord of hosts: the whole earth is full of His glory,” until post and pillar and cedar gate seemed to tremble at the sound, and the house was filled with their praise. 5T 750.1"
"In the year that king Uzziah died I saw also the Lord sitting upon a throne, high and lifted up, and his train filled the temple. Above it stood the seraphims: each one had six wings; with twain he covered his face, and with twain he covered his feet, and with twain he did fly. And one cried unto another, and said, Holy, holy, holy, is the Lord of hosts: the whole earth is full of his glory. And the posts of the door moved at the voice of him that cried, and the house was filled with smoke."
EGW directly references and describes the vision of God and the seraphim from Isaiah 6:1-4, including direct quotes from the passage.
"for us?” Isaiah with holy confidence responded, “Here am I; send me.” 5T 750.2"
"Also I heard the voice of the Lord, saying, Whom shall I send, and who will go for us? Then said I, Here am I; send me."
EGW directly quotes Isaiah's response from Isaiah 6:8.