Maliks Muwatta Book 9, Hadith Number 4.
Section : Joining Two Prayers when Settled and when Travelling.
Yahya related to me from Malik from Abu’z Zubayr al-Makki from Said ibn Jubayr that Abdullah ibn Abbas said, “The Messenger of Allah, may Allah bless him and grant him peace, prayed dhuhr and asr together and maghrib and isha together, and not out of fear nor because of travelling.” Malik said, “I believe that was during rain.”