Should one sit up before he washes neggel vasser?
You may sit up, although there is no obligation to sit up and you may wash while in a lying position. If you choose to sit up on the bed, it is best do so without placing his feet on the ground.[1]
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See Admur Basra 1:6
[1] It is our custom not to place our feet on the ground until after washing hands, as will be explained in Chapter 4 Halacha 3 Q&A