Acts 21:26
"Then Paul took the men, and the next day purifying himself with them entered into the temple, to signify the accomplishment of the days of purification, until that an offering should be offered for every one of them."
Cross References (ranked by relevance)
Acts 24:18
Whereupon certain Jews from Asia found me purified in the temple, neither with multitude, nor with tumult.
Acts 21:24
Them take, and purify thyself with them, and be at charges with them, that they may shave their heads: and all may know that those things, whereof they were informed concerning thee, are nothing; but that thou thyself also walkest orderly, and keepest the law.
1 Corinthians 9:20
And unto the Jews I became as a Jew, that I might gain the Jews; to them that are under the law, as under the law, that I might gain them that are under the law;