Musnad Ahmad 1156:77

Hajjaj narrated to us, Layth ibn Sa’d al-Masri narrated to us, he said: Yazid ibn Abi Habib narrated to me, from Abu ‘Imran Aslam, that he said: I performed Hajj with my masters, and I entered upon Umm Salamah, the wife of the Prophet, may Allah bless him and grant him peace. I said: "Should I perform ‘Umrah before I perform Hajj?" She said: "If you wish, perform ‘Umrah before you perform Hajj, and if you wish, after you perform Hajj." He said: I said: "Indeed, they say: Whoever is a *sarurah* (one who has not yet performed Hajj), it is not appropriate for him to perform ‘Umrah before he performs Hajj." He said: So I asked the Mothers of the Believers, and they said the same as what she said. I returned to her and informed her of their statement. He said: She said: "Yes, and I will satisfy you: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: 'Enter into the state of Ihram, O family of Muhammad, for an ‘Umrah within a Hajj.'"