Musnad Ahmad 33:923

Muhammad ibn Bakr narrated to us, Ibn Jurayj informed us, from Muhammad ibn al-Munkadir, from Anas ibn Malik, who said: "The Messenger of Allah (ﷺ) prayed Dhuhr in Medina as four rak'ahs, and he prayed Asr at Dhu al-Hulayfah as two rak'ahs. Then he spent the night at Dhu al-Hulayfah until morning came. When he mounted his riding beast and it stood upright with him, he began the talbiyah."