Which hand do Muslims believe is the dirty hand?

Answer 1

There is no dirty hand, its just known that the right hand is heaven

and the left hand is hell.

Ayatullah Khomeinie speeks about Imam Ali a.s doing withuu.

Ameer al muumineen a.s will pick up the water he knows its the spring of life and knows how clear it is so he'd be clear with Allah as well, when he washes his face he'd say ya Allah let Noor shine on this face on the day of judgment, then when he puts the water on his right arm he'd say Ya Allah let me be the people on the right hand (heaven) on the day of judgment then when he lift the water and put it on his left arm he'd say ya Allah do not let me be the people on the left and on the day of judgment.

Then when he wiped his head he's say Ya Allah allow this head to bow down to you on the day of judgment, the when he wiped his feet he'd say On a day when all the feet are slipping, let my feet not slip on the day of judgment