Is it permissible for a husband to take back his wife after a divorce without witnesses?

If the divorce is revocable, and you were taken back before your 'iddah (waiting period) ended, then the revocation is valid, and having witnesses to it is recommended. The divorce is revocable if it occurred after consummation, and it was not the third divorce, and it was not in exchange for compensation, and it was not ruled by a judge. The revocation is effected simply by the husband saying: "I take back my wife," or by sexual intercourse. The validity of the revocation does not depend on the wife's consent or knowledge.