Hasan al-Basri, Ibrahim al-Nakha'i, and al-Sha'bi, may Allah have mercy on them, said: If someone gives his wife one or two divorces and has intercourse with her during the waiting period, then reconciliation has taken place, and it is necessary to bear witness.
Hadith Referenceسنن سعید بن منصور / كتاب الطلاق / 2499
Hadith Takhrij«أخرجه سعيد بن منصور فى «سننه» ترقيم الدرالسلفية برقم:، 1322، وابن أبى شيبة فى «مصنفه» برقم: 18072، 18074»