Sahih Muslim Book 6, Hadith Number 2629.
Chapter : Excellence of Lailat-ul Qadar and exhortation to find it.
This Hadith has been reported on the authority of Yahya b. Abu Kathir with the same chain of transmitters (with a slight variation of these words): I saw the Messenger of Allah (may peace be upon him) after he had completed (the prayer) and there was a trace of clay on his forehead and tip (of the nose).