I prayed beside Ibn Umar and placed my hand on my waist. When he finished praying, he said, "This is crucifixion in prayer, and the Messenger of Allah ﷺ forbade it."
صليت إلى جنب ابن عمر، فوضعت يدي على خاصرتي، فلما صلى قال: هذا الصلب في الصلاة، وكان رسول الله ﷺ ينهي عنه.
Abu Dawud (903) and Nasai (892) both narrated it via Saeed bin Ziyad, from Ziyad bin Subayh, who related the same account. The wording given is that of Abu Dawud. In Nasai's version, he said, "I prayed next to Ibn Umar, and I placed my hand on my hip, whereupon he struck it away from me with his hand, like this. When I had finished praying, I asked a man, 'Who is that?' He said, 'Abdullah bin Umar.' I said, 'Abu Abdul-Rahman, what did you object to in me?' He said, 'This is the crucifixion posture, and the Messenger of Allah ﷺ forbade us from it.'"Imam Ahmad (4849) also narrated something similar. Its chain of transmission is hasan, because Saeed bin Ziyad Shaybani is disputed. He was deemed trustworthy by Ibn Main, Ajli, and Ibn Hibban, while Nasai said, "There is no harm in him."There is disagreement over the meaning of "takhassur." The correct view held by the scholars of hadith is that it means a man prays while placing his hand on his hip. There is also disagreement over the wisdom behind the prohibition. The correct view is that it resembles the shape of a cross, as Abdullah bin Umar stated.Ayesha disliked placing one's hand on the hip, and used to say, "The Jews do that." This was recorded by Bukhari in the chapters on the accounts of the prophets (3458), as her own statement rather than one attributed to the Prophet.Khattabi said, "This is something the Jews do." It has also been reported in some accounts that Iblis was cast down to earth in that posture, and that it resembles the posture of people afflicted by calamity, who place their hands on their hips when they stand at funeral gatherings. It has also been said that it refers to holding a "mikhsara" in one's hand, that is, a staff one leans on.Ibn Hajar said, "There is disagreement over the wisdom behind this prohibition. Some of what I mentioned has been cited as reasons for it, and other reasons besides these have also been added." Fath al-Bari (3/89).