Ar-Raj'ah Without Calling Witnesses is Correct
Question :
A man married a woman and after some time, he divorced her, then later, he took her back without witnesses to that Raj'ah, and his wife warned him against approaching her, fearful of committing a forbidden act. But he said that he had called Allah to witness it and Allah is Sufficient as a Witness. Is this permissible?
Answer:
When a man divorces his wife and then takes her back before the expiry of her waiting period, the Raj'ah is valid, and the wife returns to his authority.
As for calling witnesses to the Rajah, scholars are divided on that. Some of them hold the view that it is obligatory, while others said that it is a Sunnah. But it appears that it is not obligatory and that it is only a Sunnah; based upon this, the wife returns to her husband, if he takes her back during the waiting period, whether he calls witnesses or not. Although the most complete Raj'ah is to call witnesses.
As for the wife warning to her husband due to fear of doing something forbidden, I can reassure her that it is not forbidden, Allah Willing. As for the saying of the man that he called Allah to witness it, Allah is a Witness over all things, but the testimony which Allah has commanded is that we have just witnesses from among us.
Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 6 Page 70