"When the Prophet would prostrate, he placed his nose and his forehead on the ground, and he held his forearms away from his sides, and he placed his hands parallel to his shoulders."
Jami` at-Tirmidhi Book 2, Hadith 122
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Jami` at-Tirmidhi Book 2, Hadith 121 Next Hadith → "I said to Al-Bara bin Azib: 'Where would the Prophet place his face when he prostrated?' He said: 'Between his hands.'"

Jami` at-Tirmidhi Book 2, Hadith 123