It is narrated from Ziyad bin Jubair that Sayyiduna Ibn Umar (may Allah be pleased with them both) saw a man who had made his camel sit down (for sacrifice). He said, "Make it stand up and tie it with a rope, then perform nahr (i.e., slaughter it), because this is the Sunnah of Muhammad (peace and blessings be upon him)."
Hadith Referenceسنن دارمي / من كتاب المناسك / 1952
Hadith Takhrijاس روایت کی سند صحیح اور حدیث متفق علیہ ہے۔ دیکھئے: [بخاري 1713] ، [مسلم 1320] ، [أبوداؤد 1768] ، [ابن حبان 5903]
Brief Explanation
(Commentary on Hadith 1951)
From this hadith, it is understood that performing nahr (slaughtering) of a camel while it is standing is superior. Imam Abu Hanifah rahimahullah said that it is also permissible to perform nahr while the camel is made to sit.