"The Prophet would perform Salat in sheep pens."
Jami` at-Tirmidhi Book 2, Hadith 202
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Jami` at-Tirmidhi Book 2, Hadith 201 Next Hadith → "The Prophet dispatched me for something he needed. I came to him while he was performing Salat on his mount, heading east, he made the prostrations lower than the bowing."

Jami` at-Tirmidhi Book 2, Hadith 203