Sahih Bukhari Volume 009, Book 090, Hadith Number 345.
Narrated By 'Ata : One night the Prophet delayed the Isha' prayer whereupon 'Umar went to him and said, "The prayer, O Allah's Apostle! The women and children had slept." The Prophet came out with water dropping from his head, and said, "Were I not afraid that it would be hard for my followers (or for the people), I would order them to pray Isha prayer at this time." (Various versions of this Hadith are given by the narrators with slight differences in expression but not in content).