اعْتَدِلُوا فِي السُّجُودِ، وَلَا يَبْسُطْ أَحَدُكُمْ ذِرَاعَيْهِ انْبِسَاطَ الْكَلْبِ
Be balanced in prostration, and let none of you spread his forearms out as a dog spreads its limbs.
Source: البخاري ومسلم
اعْتَدِلُوا is a plural imperative meaning “be balanced.” In لَا يَبْسُطْ, لَا expresses prohibition and the verb means “spread out.” ذِرَاعَيْهِ is dual: “his two forearms.”
Prostration calls for composure and a proper, balanced position. Spreading the forearms flat in the compared manner is the specific posture cautioned against. Physical etiquette in prayer supports attentiveness and reverence; it is not simply a matter of external appearance.