I prayed beside Ibn Umar and placed my hand on my waist. When he had finished the prayer, he said, "This is crucifixion during prayer, and the Messenger of Allah ﷺ used to forbid it."
حدثنا هناد بن السري عن وكيع عن سعيد بن زياد
صليت إلى جنب ابن عمر فوضعت يدي على خاصرتي فلما صلى قال هذا الصلب في الصلاة وكان رسول الله ﷺ ينهى عنه
Reference, its chain of transmission is sahih. Nasai narrated it in the book on the Opening of Prayer, chapter on the prohibition of placing the hand on the hip during prayer, hadith no. 892, from the hadith of Saeed bin Ziyad, with the same wording.