Musnad Abu Ya'la al-Mawsili 2:34

Abdullah ibn Umar ibn Aban narrated to us, Yahya ibn Zakariya ibn Abi Za’idah narrated to us, from Salih ibn Abi Salih, from Salamah ibn Kuhayl, from Sa’id ibn Jubayr, from Ibn Abbas, from Umar: That the Prophet, may Allah bless him and grant him peace, had divorced Hafsa, then he took her back.