Musnad Ahmad 32:1479

Ishaq ibn Mansur—meaning al-Saluli—narrated to us, Ammarah—meaning ibn Zadhan—narrated to us, from Thabit, from Anas, who said: The Prophet, peace and blessings of Allah be upon him, used to take a midday nap at the home of Umm Sulaym. He was among the people who sweated the most, so she prepared a leather mat for him, and he would take his midday nap upon it. She stitched a line between his legs, and she would soak up the sweat and collect it. He said, "What is this, O Umm Sulaym?" She replied, "Your sweat, O Messenger of Allah; I put it in my perfume." So he prayed a good prayer for her.