Maliks Muwatta Book 1, Hadith Number 20.
Section : Duluk ash-Shams and Ghasaq al-Layl.
Yahya related to me from Malik that Da’ud ibn al-Husayn said that someone had told him Abdullah ibn Abbas used to say, “Duluk ash-shams begins from when the sun passes the meridian. Ghasaq al-layl is the gathering of the night and its darkness.”