Sahih Muslim Book 4, Hadith Number 1335.

Chapter : Commencement of the time for the evening prayer is immediately after sunset.

Abdullah b. ‘Umar reported that the Messenger of Allah (may peace be upon him) was one night occupied (in some work) and he delayed it (‘Isha prayer) till we went to sleep in the mosque. We then woke up and again went to sleep and again woke up. The Messenger of Allah (may peace be upon him) then came to us and said: None among the people of the earth except you waits for prayer in the night.