Sahih Muslim Book 004, Hadith Number 1339.

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

Abu Musa reported: I and my companions who had sailed along with me in the boat landed with me in the valley of Buthan while the Messenger of Allah (may peace be upon him) was staying in Medina. A party of people amongst them went to the Messenger of Allah (may peace be upon him) every night at the time of the 'Isha prayer turn by turn. Abu Musa said: (One night) we (I and my companions) went to the Messenger of Allah (may peace be upon him) and he was occupied in some matter till there was a delay in prayer so much so that it was the middle of the night. The Messenger of Allah (may peace be upon him) then came out and led them (Musa's companions) in prayer. And when he had observed his prayer he said to the audience present: Take it easy, I am going to give you information and glad tidings that it is the blessing of Allah upon you for there is none among the people, except you, who prays at this hour (of the night), or he said: None except you observed prayer at this (late) hour. He (i.e. the narrator) said: I am not sure which of these two sentences he actually uttered. Abu Musa, said: We came back happy for what we heard from the Messenger of Allah (may peace be upon him).

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