He (PBUH) used to teach them to perform a ritual bath after becoming Muslim

Qays ibn ‘Aasim (a Companion) reported that after he became Muslim, the Prophet of Allah, sallallaahu ‘alayhi wa sallam, ordered him to bathe with water mixed with lote leaves

Abu Hurayrah narrated that Thumaamah ibn Uthaal (a tribal chieftain) became Muslim, so the Prophet of Allah, sallallaahu ‘alayhi wa sallam, said: “Take him to the garden of so and so and order him to bathe