Sahih Muslim 1320
Sahih Muslim · Book 15: The Book of Pilgrimage · It is recommended to sacrifice the camel when it is standing and tied
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى ، أَخْبَرَنَا خَالِدُ بْنُ عَبْدِ اللَّهِ ، عَنْ يُونُسَ ، عَنْ زِيَادِ بْنِ جُبَيْرٍ ، أَنَّ ابْنَ عُمَرَ ، أَتَى عَلَى رَجُلٍ وَهُوَ يَنْحَرُ بَدَنَتَهُ بَارِكَةً فَقَالَ ابْعَثْهَا قِيَامًا مُقَيَّدَةً سُنَّةَ نَبِيِّكُمْ صلى الله عليه وسلم .
Ziyad b. Jubair reported that Ibn 'Umar came upon a person who was slaughtering (sacrificing) his camel and had made him kneel down. So he told him to make it stand up festered (and then sacrifice it) according to the Sunnah of the Prophet (ﷺ).