Musnad Ahmad 1153:270

Yahya narrated to us, from Ibn Jurayj, who said: I heard ‘Ata’ saying: ‘Urwah ibn al-Zubayr informed me, saying: I and Ibn ‘Umar were leaning against the chamber of ‘A’ishah, and we were hearing her using the miswak (tooth-stick). I said: "O Abu ‘Abd al-Rahman, did the Prophet, may Allah bless him and grant him peace, perform ‘Umrah in Rajab?" He said: "Yes." I said: "O Mother, do you hear what Abu ‘Abd al-Rahman is saying?" She said: "What is he saying?" I said: "He says: 'The Prophet, may Allah bless him and grant him peace, performed ‘Umrah in Rajab.'" She said: "May Allah forgive Abu ‘Abd al-Rahman; he has forgotten. The Prophet, may Allah bless him and grant him peace, did not perform ‘Umrah in Rajab." He said: And Ibn ‘Umar was listening, and he did not say no, nor did he say yes; he remained silent.