Shama-il Tirmidhi Chapter 019, Hadith Number 001 (119).

Hazrat Anas bin Malik radiyallahu anhu says, "Rasoollullah sallallahu alaihe wasallam often wore a cloth on his auspicious head. This cloth, because of its greasiness, looked as if had been oiled."

Commentary.
That means that the cloth was oily due to the excessive use of oil. Besides all this, it is a peculiarity of Sayyidina Rasoollullah sallallahu alaihe wasallam that this cloth did not become dirty, nor did any lice live in the clothing of Sayyidina Rasoolullah sallallahu alaihe wasallam, nor could an insect suck his auspicious blood. Allamah Munaawi has related from (Qari) Allamah Raazi, "A fly never sat on the clothing of Sayyidina Rasoolullah sallallahu alaihe wasallam."