Musnad Ahmad 658:221

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Zubayd al-Yamani, from al-Sha'bi, from al-Bara' ibn 'Azib, who said: The Messenger of Allah (ﷺ) said: "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 slaughters [before the prayer], it is merely meat that he has presented to his family; it is not part of the ritual sacrifice (nusuk) in any way." He said: Abu Burdah ibn Niyar had already slaughtered, so he said: "I have a young goat (jadha'ah) that is better than an old sheep (musinnah)." He (the Prophet) said: "Slaughter it, but it will not suffice for anyone after you."