Musnad Ahmad 658:14

'Affan narrated to us, Shu'bah narrated to us, he said: Zubayd, Mansur, Dawud, Ibn 'Awn, and Mujalid informed me, from al-Sha'bi—and this is the hadith of Zubayd—he said: I heard al-Sha'bi narrating from al-Bara', and he narrated to us by a pillar in the mosque, and he said: "If I were there, I would have informed you of its location." He said: The Messenger of Allah (ﷺ) delivered a sermon to us and said: "Indeed, the first thing we begin with on this day of ours is that we pray, then we return and slaughter. Whoever does that has attained our Sunnah, and whoever slaughtered before that, it is merely meat he has presented to his family; it is not part of the ritual sacrifice (nusuk) in any way." He said: My maternal uncle, Abu Burdah ibn Niyar, slaughtered and said: "O Messenger of Allah, I have slaughtered, and I have a young goat (jadha'ah) that is better than an old one (musinnah)." He said: "Make it take its place, but it will not suffice—or fulfill—for anyone after you."