Musnad Ahmad 788:50

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Qays ibn Muslim, from Tariq ibn Shihab, from Abu Musa, who said: I arrived to the Prophet, may God bless him and grant him peace, while he was camping at al-Abtah. He said to me: "Have you performed Hajj?" I said: Yes. He said: "With what did you enter into the state of Ihram?" I said: I said, "Labbayk (I am here) with the same Ihram as the Ihram of the Prophet, may God bless him and grant him peace." He said: "You have done well." He said: "Perform Tawaf around the House, and at al-Safa and al-Marwah, then exit the state of Ihram." He said: So I performed Tawaf around the House, and at al-Safa and al-Marwah, then I went to a woman from Banu Qays who combed my hair, then I entered into the state of Ihram for Hajj. He said: I used to give this fatwa to the people. Until it was the caliphate of Umar, may God be pleased with him, and a man said: "O Abu Musa—or O Abdullah ibn Qays—slow down regarding some of your fatwas; for you do not know what the Commander of the Believers has introduced regarding the rituals after you." He said: So he said: "O people, whoever we have given a fatwa to, let him wait; for the Commander of the Believers is coming to you, so follow him." He said: Then Umar arrived, and I mentioned that to him. He said: "If we take the Book of God, the Book of God Almighty commands us to complete [the Hajj and Umrah], and if we take the Sunnah of the Messenger of God, may God bless him and grant him peace, then the Messenger of God, may God bless him and grant him peace, did not exit the state of Ihram until the sacrificial animal reached its place."