Purity: Purification Upon Conversion to Islam After Declaring the Shahādatayn
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 205
If a disbeliever declares in any language the shahādatayn (two testimonies) – i.e. he testifies to the oneness of Allah the Exalted and to the prophethood of the Seal of the Prophets [Prophet Muḥammad (Ṣ)] – he becomes a Muslim. In the event that he was previously ruled to be impure based on obligatory precaution,[39]1 then, after becoming a Muslim, his body, saliva, nasal mucus, and sweat are pure. However, if at the time of becoming a Muslim an intrinsic impurity is on his body, it must be removed and that part of his body must be washed; and if the intrinsic impurity is removed before he becomes a Muslim, then based on obligatory precaution, that part of his body must be washed.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)
[1] See Rulings 104, 105, and 107.