He said it repeatedly in John 6. Isn’t that “repeatedly” enough for you?
Some things; said once; should be sufficient.
Jesus answered, The work of God is this: to believe in the one he has sent.
One discourse..Arthur ..one discourse.. not repeated in any other gospel ... He also said the following
28 Then said they unto him, What shall we do, that we might work the works of God?
29 Jesus answered and said unto them, This is the work of God, that ye believe on him whom he hath sent.
40 And this is the will of him that sent me, that every one which seeth the Son, and believeth on him, may have everlasting life: and I will raise him up at the last day.