Hadith 10:50
Abu Bakr narrated to us, he said: Muhammad ibn Fudayl narrated to us, from al-Shaybani, from al-Sha'bi, that he was asked about a man who divorced his wife, then took her back, but was ignorant of [the requirement] to call witnesses. He said: "He shall call witnesses when he learns [of the requirement]."
حدثنا أبو بكر قال نا محمد بن فضيل عن الشيباني عن الشعبي أنه سئل عن رجل طلق امرأته ثم راجعها فيجهل أن يشهد قال يشهد إذا علم