Ibn Jurayj narrates from Ata' (may Allah have mercy on him) that for taking back (a divorced wife) two witnesses are required; if there is an excuse, then one witness and one oath.
Imam al-Shafi'i (may Allah have mercy on him) said: Ata' (may Allah have mercy on him) gave a ruling by an oath and a witness in a case in which none of us holds that view, and he said:
Imam al-Shafi'i (may Allah have mercy on him) said: An oath along with a witness is not contrary to the apparent meaning of the Qur'an, because we judge by two witnesses, and by one witness and two women, with no oath; and when there is one witness we judge by a witness and an oath. This is not contrary to the apparent meaning of the Qur'an, because it did not forbid that less than what it explicitly stated in His Book be valid.
Imam al-Shafi'i (may Allah have mercy on him) said: The Messenger of Allah (peace and blessings be upon him) knew best the meaning of what Allah, the Mighty and Majestic, intended, and Allah, the Mighty and Majestic, has commanded us to take what he gives us and to refrain from what he forbids us. We ask Allah for protection and success.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20690