Why do some Muslims pray with their arms by their sides?

Two ways of standing

If you visit a mosque in London, Paris or New York, you'll notice people standing in two ways after the opening takbir:

• Qabd (folding): right hand placed over the left, on the chest, stomach or below the navel. This is the practice of the Hanafi, Shafi'i and Hanbali schools.
• Sadl or irsal (arms down): hands hanging relaxed at the sides. This is the well-known Maliki practice, and also the practice of Shia Muslims and the Ibadis of Oman.

Neither group is doing something "wrong". These are differences in how scholars understood the evidence, and Muslims have prayed side by side with both for centuries.

The evidence for folding the hands

Sahl ibn Sa'd said that people were instructed to place the right hand on the left forearm in prayer (Sahih al-Bukhari 740). Wa'il ibn Hujr described seeing the Prophet ﷺ raise his hands, say the takbir, then place his right hand over his left (Sahih Muslim 401). Because of these hadiths, the majority of scholars, and many Malikis too, say folding is the sunnah.

Why Malikis pray with arms down

Imam Malik lived in Medina and gave great weight to the practice of its people, which he saw as inherited from the Prophet's ﷺ city. A narration in his Mudawwana reports that he disliked folding the arms in obligatory prayers, especially if it was done to lean or rest. Later Maliki scholars explained his reasons differently, and today some Malikis fold and many let their arms hang. In Shia jurisprudence, letting the arms hang is the standard position as well.

Both sides are trying to follow that pattern as closely as they understand it.

What should I do as a new Muslim?

Follow what your teacher or local community does, or the majority practice of folding if you're unsure. What matters far more is that you pray, recite al-Fatiha, and do each position calmly. See How do I pray as a Muslim?

If you pray next to someone with their arms down, don't correct them. It's not a sign of a "wrong" sect, and pointing it out can hurt. Many converts worry about choosing a group at all. See Do I have to choose Sunni or Shia to become Muslim?