Sahih Muslim Book 4, Hadith Number 1334.

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

Abdullah b. Umar reported: We waited one night in expectation of the Messenger of Allah (may peace be upon him) for the last prayer of the night, and he came out to us when a third of the night had passed even after that. We do not know whether he had been occupied with family business or something else. When he came cut he said: You are waiting for prayer, for which the followers of no other religion wait, except you. Were it not a burden for my Ummah, I would have led them (in the ‘Isha prayer) at this hour. He then ordered the Mu’adhdbin (to call for prayer) and then stood up for prayer and observed prayer.