All praise is due to Allah. May Allah's peace and blessings be upon the Messenger of Allah, his family and his companions.
A revocable divorce is when the woman is taken back by her husband during her waiting period without contracting a new marriage, dowry (mahr), or the consent of the wife, if the man takes back his wife with the intention of normalizing things. The waiting period for a woman who menstruates is three menstrual periods. The waiting period for the woman who does not menstruate is three months. When taking back his wife, the man says, "I have taken back my wife so and so," in the presence of witnesses. It is not compulsory to have two witnesses present when taking back the wife. If a person witness the tacking back, and another one witnesses later, the Sunnah would have been fulfilled.
Shaykh Khalid al-Mosleh