Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) returned Sayyidah Zaynab (may Allah be pleased with her) to Abu al-As upon the first marriage and did not do anything new. There is also a difference of opinion about this, and some say: with a new marriage contract.
Hadith Referenceالمعجم الكبير للطبراني / باب اللام / 16504