Performing Wudhu before sleeping is the sunnah of Rasool-Allah Sallallahu Alaihi Wasallam

Bismillahirrahmanirraheem

✦ Perform Wudhu before sleeping is the sunnah of Rasool-Allah Sallallahu Alaihi Wasallam   

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✦ Narrated Aisha Radi Allahu Anha Whenever the Rasool-Allah Sallallahu Alaihi Wasallam   intended to sleep he used to perform ablution like that for the prayer

Musnad Ahmed-5512-Sahih