Shama-il Tirmidhi Chapter 007, Hadith Number 002 (049).

Ibn 'Abbaas Radiyallahu 'Anhu reports: "Rasulullah Sallallahu 'Alayhi Wasallam applied kuhl of ithmid three times in each eye before sleeping'. In a narration also from Ibn 'Abbaas Radiyallahu 'Anhu, it is reported: "Rasuluilah Sallallahu'Alayhi Wasallam had a small container for keeping kuhl, from which he applied in each eye kuhl three times before sleeping'.