The Kauthar (A river of Prophet Muhammed (pbuh) in Paradise)

Anas (R.A) narrated: Prophet Muhammad (pbuh), one day took a nap among us and woke up laughing. We asked him, "O messenger of Allah why are you laughing"
The Prophet (pbuh) said, "A surah was revealed to me before"
Then he read out, "In the name of Allah, the most beneficent, the most beautiful"

"Verily, we have granted you (O Muhammed) Al-Kauther (a river in Paradise). Therefore, turn in prayer to your lord and sacrifice (to him only). For, he who makes you angry (O Muhammed) will be cut off" ( Al-kauther )

Then the Prophet (pbuh) asked, "Do you know what Al-kauther is?
His companions said, "Allah and his Messenger knows best"

Then the Prophet (pbuh) said, "It is a river in Paradise Allah pledged to give me. It is full of every good thing and my ummah will aim at it on the Day of Resurrection. It's cups are as much as stars. Some of my followers will be prevented from it and I will ask O my lord they are my followers.
Allah will say, "you (O Muhammad) do not know how they heterodoxy (after you).

(This Hadith is sound and reported by, Muslim and Abu Da'ud)

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This Hadith reflects the danger of deviation from the Sunnah of the Prophet (pbuh). Every good deed has two conditions to be accepted by Allah: One is that it should be solely for Allah without any partner whatsoever. Second condition that is should be performed according to the Sunnah.