Hadith: I saw Ibn ‘Umar passing by a man who had made his camel kneel down so as to slaughter it. Ibn ‘Umar said: Slaughter it while it is standing with one leg tied up, as this is the Sunnah of Muhammad (may Allah's peace and blessings be upon him)

عن زياد بن جبير قال: رَأَيتُ ابنَ عُمَرَ أَتَى عَلَى رجل قد أَنَاخَ بَدَنَتَهُ، فَنَحَرَهَا، فَقَالَ: ابْعَثْهَا قِيَاماً مُقَيَّدَةً، سُنَّةَ مُحَمَّدٍ -صلَّى الله عليه وسلَّم-.
[صحيح] - [متفق عليه]

Ziyād ibn Jubayr reported: I saw Ibn ‘Umar passing by a man who had made his camel kneel down so as to slaughter it. Ibn ‘Umar said: "Slaughter it while it is standing with one leg tied up, as this is the Sunnah of Muhammad (may Allah's peace and blessings be upon him)."
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

The Sunnah regarding the slaughter of cows, sheep, and others is to cut their throats while they are lying on their left sides, facing the Qiblah. As for a camel, then the Sunnah in slaughtering it is to cut the jugular vein in its neck while it is standing with its left leg tied. That is because this position is comfortable for the camel by letting its soul depart the body quickly. For this reason, when ‘Abdullāh ibn ‘Umar (may Allah be pleased with him) passed by a man who wanted to slaughter his camel while kneeling down, he said: Slaughter it while it is standing with one leg tied up, as this is the Sunnah of the Prophet (may Allah's peace and blessings be upon him) who acted upon the instructions of the Qur'an on slaughter as Allah, the Almighty, says: {when they are down on their sides (after slaughter)} [Sūrat al-Hajj: 36]. i.e. fall down, and falling down occurs only when it is in a standing position.