Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.020.

Section : One should not clean himself with his right hand.

‘Abdurahman ibn Zaid related that Salman was asked, “Your Prophet teaches you everything, even how to relieve yourselves?” Salman said, “Certainly… He forbade us from facing the qiblah while doing so, from cleaning ourselves with our right hand, and from cleaning ourselves with less than three stones. We also should not use an impure substance or a bone to clean ourselves.” (Related by Muslim, Abu Dawud, and at-Tirmizhi.)

Hafsah reported, “The Messenger of Allah, upon whom be peace, reserved his right hand for eating, drinking, putting on his clothes, taking and giving. He used his left hand for other actions.” (Related by Ahmad, Abu Dawud, Ibn Majah, Ibn Hibban, al-Hakim and al-Baihaqi).