Imam al-Shafi'i (may Allah have mercy on him) said: "She is the wife of the first (husband), (because) the Messenger of Allah (peace and blessings be upon him) said: 'When two guardians (separately) give (a woman) in marriage, the first (marriage) has the greater right.'"
This hadith has already passed with its chains.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرجعة / Q15187
Sa'id bin Jubayr narrates from Sayyiduna Ali (may Allah be pleased with him) concerning a man who divorced his wife, then made witnesses to his taking her back, but the woman did not know of it. He (may Allah be pleased with him) said: She is the wife of the first, whether the other has consummated the marriage with her or not.
(6) Chapter: What has been narrated about taking witnesses to the taking back (raj'ah) "The chapter on making witnesses to taking back"
Allah Almighty said: "Then retain them in a good manner or part with them in a good manner, and take as witnesses two just men from among you." [Surah al-Talaq: 2]
The statement of Allah Almighty: "Retain them with goodness or separate from them with kindness, and make two people of justice witnesses." [Surah al-Talaq: 2]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرجعة / 15187
Ubaydullah narrates from Nafi' that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) gave his wife Safiyyah bint Abi Ubayd one or two divorces, so he used to go to her (only) after seeking permission; and when he took her back and made witnesses to it, he would then go to her without permission.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرجعة / 15188
Ibn Sirin (may Allah have mercy on him) says that Imran bin Husayn (may Allah be pleased with him) was asked about a man who divorced his wife without making witnesses and took her back, also without making witnesses. Imran bin Husayn (may Allah be pleased with him) said: He divorced outside the (proper) waiting period and took her back in a way other than the Sunnah; let him make witnesses now.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرجعة / 15189