سنن النسائي

It was narrated that 'Amir bin 'Abdullah said: "I saw the letter of 'Umar bin Al-Khattab to Abu Musa (in which he said): 'A caravan came to me from Ash-Sham carrying a thick black paint like the pitch that is daubed on camels. I asked them how long they cooked it, and they told me that they cooked it until it was reduced by two-third. So the bad two-third had gone, one-third to take away evil and one-third to take away the bad smell. So let those who are with you drink it."