Sahih Muslim Book 4, Hadith Number 1523.

Chapter : Permissibility of combining two prayers on a journey.

‘Abdullah b. Shaqiq reported: Ibn ‘Abbas one day addressed us in the afternoon (after the afternoon prayer) till the sun disappeared and the stars appeared, and the people began to say: Prayer, prayer. A person from Banu Tamim came there. He neither slackened nor turned away, but (continued crying): Prayer, prayer. Ibn ‘Abbas said: May you be deprived of your mother, do you teach me Sunnah? And then he said: I saw the Messenger of Allah (may peace be upon him) combining the noon and afternoon prayers and the sunset and ‘Isha prayers. ‘Abdullah b. Shaqiq said: Some doubt was created in my mind about it. So I came to Abu Huraira and asked him (about it) and he testified his assertion.