Sahih Bukhari Volume 001, Book 010, Hadith Number 544.
Narated By Ibn Shihab from 'Urwa : 'Aisha said, "Once Allah's Apostle delayed the 'Isha' prayer till 'Umar reminded him by saying, "The prayer!" The women and children have slept. Then the Prophet came out and said, 'None amongst the dwellers of the earth has been waiting for it (the prayer) except you." Urwa said, "Nowhere except in Medina the prayer used to be offered (in those days)." He further said, "The Prophet used to offer the 'Isha' prayer in the period between the disappearance of the twilight and the end of the first third of the night."