Hadith 8:3026
Abu Bakr narrated to us, he said: Abu Khalid narrated to us, from Hajjaj, from Ata, that Ibn Umar used to slaughter them while standing, while he was young. When he grew old, he slaughtered them while they were kneeling.
حدثنا أبو بكر قال نا أبو خالد عن حجاج عن عطاء أن بن عمر كان ينحرها شابا قياما فلما كبر نحرها وهي باركة