The Muslim custom is to use the left hand as toilet paper.
Some of their customs are just so quaint. They really should give the 21st century a try. There is much to offer, such as actual toilet - paper.
I'd hardly call it explicitly a Muslim custom - it's common practice throughout south Asia; many Hindus and Buddhists do that as well. It's more of a result of the (historically) extreme poverty in the region, coupled with the arid climate which means that there's often a lack of plant products which are often used in areas of similar poverty but with wetter climates.
However, the fact that the left hand is often used for "personal hygiene" means that various cultural practices have become established - such as never offering food with one's left hand, etc. Again, this is not so much a religious custom so much as it is a regional custom, and is practiced by Hindus and Buddhists as well ... though because of Islam's place of origin the custom is often adopted into the religion and brought along with Islam to places where the orignal reason for the custom is no longer relevant. Hinduism for example does not seek converts so the issue never comes up outside its place of origin.
Bruce