Ignatius of Antioch, writing about AD 107, affirmed that Jesus was conscience during the three days when his body was in the tomb, an early witness against soul sleep/death:
Now, He suffered all these things for our sakes, that we might be saved. And he suffered truly, even as also He truly raised up Himself, not, as certain unbelievers maintain, that He only seemed to suffer, as they themselves only seem to be Christians. And as they believe, so shall it happen unto them, when they shall be divested of their bodies, and be mere evil spirits. (To the Smyrnaeans, 2:1)
The underlying Greek for the section in bold is ἀληθῶς ἀνέστησεν ἑαυτόν. The adverb ἀληθῶς means "truly"; ἀνέστησεν is the indicative aorist active third-person singular of the verb ἀνίστημι ("to cause to arise"/"to arise"); as it is in the active, not passive, this means that this is something Jesus Himself did, and this is strengthened by it being coupled with the reflexive pronoun ἑαυτόν ("of himself"), again, stressing how it was Jesus who raised himself from the dead in Ignatius’s theology.
For more on the dead being conscious in the intermediate state, see:
