Musnad Ahmad 2:257

Sufyan narrated to us, from Yahya—meaning Ibn Sa'id—from Ubayd ibn Hunayn, from Ibn Abbas, who said: I wanted to ask Umar, but I did not find an opportunity, so I waited for two years. When we were at Marr al-Zahran, he went to relieve himself. He came back having relieved himself, and I went to pour water for him. I said: "O Commander of the Faithful, who were the two women who conspired against the Messenger of Allah, may Allah bless him and grant him peace?" He said: "Aisha and Hafsa."