"Sickness, suffering, and death are work of an antagonistic power. Satan is the destroyer; God is the restorer. MH 113.1 The words spoken to Israel are true today of those who recover health of body or health of soul. “I am the Lord that healeth thee.” Exodus 15:26 . MH 113.2"
"And said, If thou wilt diligently hearken to the voice of the Lord thy God, and wilt do that which is right in his sight, and wilt give ear to his commandments, and keep all his statutes, I will put none of these diseases upon thee, which I have brought upon the Egyptians: for I am the Lord that healeth thee."
Ellen White directly references Exodus 15:26 in this passage.
"He it is who “forgiveth all thine iniquities; who healeth all thy diseases; who redeemeth thy life from destruction; who crowneth thee with loving-kindness and tender mercies.” Psalm 103:3, 4 . MH 113.4"
"Who forgiveth all thine iniquities; who healeth all thy diseases;"
EGW directly quotes Psalm 103:3.
"Who redeemeth thy life from destruction; who crowneth thee with lovingkindness and tender mercies;"
EGW directly quotes Psalm 103:4.
"And said, If thou wilt diligently hearken to the voice of the Lord thy God, and wilt do that which is right in his sight, and wilt give ear to his commandments, and keep all his statutes, I will put none of these diseases upon thee, which I have brought upon the Egyptians: for I am the Lord that healeth thee."
Exodus 15:26 establishes a clear link between obedience to God and health, which is the principle EGW is referencing.