Sahih Muslim Book 4, Hadith Number 1337.

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

Anas b. Malik reported: We waited for the Messenger of Allah (may peace be upon hi n) one night, till it was about midnight. He (the Holy Prophet) came and observed prayer and then turned his face towards us, as it I was seeing the lustre of the silver ring on his finger.