Baptism was normatively concomitant with regeneration in Scripture, as it can be the time when one confessed Jesus is Lord out of the justifying faith, baptism being a sinners prayer in body language. But the most explicit text which shows regeneration occurring before baptism is Acts 10:43-47:
“To him give all the prophets witness, that through his name whosoever believeth in him shall receive remission of sins. While Peter yet spake these words, the Holy Ghost fell on all them which heard the word. And they of the circumcision which believed were astonished, as many as came with Peter, because that on the Gentiles also was poured out the gift of the Holy Ghost. For they heard them speak with tongues, and magnify God. Then answered Peter, Can any man forbid water, that these should not be baptized, which have received the Holy Ghost as well as we?”
cf. Acts 15:7-9:
“And when there had been much disputing, Peter rose up, and said unto them, Men and brethren, ye know how that a good while ago God made choice among us, that the Gentiles by my mouth should hear the word of the gospel, and believe. And God, which knoweth the hearts, bare them witness, giving them the Holy Ghost, even as he did unto us; And put no difference between us and them, purifying their hearts by faith.”
And there we have it, from Peter's own lips.