Musnad Ahmad 30:2920

Muhammad ibn Ja'far narrated to us, he said: Shu'bah narrated to us, and Hajjaj said: Shu'bah narrated to me, from Qatadah, from an-Nadr ibn Anas, from Bashir ibn Nahik, from Abu Hurairah, from the Prophet, may Allah bless him and grant him peace, that he said: "Al-'Umra is permissible."