Abu Dawud

355. Commentary:
A ritual bath (ghusl) is obligatory for a new Muslim who accepts Islam. [عون المعبود]

188. Commentary:

➊ This ritual bath (ghusl) is considered recommended (mustahabb) according to the majority of scholars, so that the person feels that he has become purified from both internal and external impurity and filth. In fact, according to some narrations, shaving the hair and circumcision are also commanded. The Prophet sallallahu alayhi wa sallam instructed Kulaib radi Allahu anhu, when he accepted Islam: «ألق عنک شعر الکفر» "Remove the hair of disbelief from yourself." The Prophet sallallahu alayhi wa sallam also instructed another companion: «ألق عنک شعر الکفر و اختتن» "Remove the hair of disbelief from yourself (i.e., get your hair shaved) and perform circumcision." [سنن أبي داود ، الطھارۃ ، حدیث : 383]

Shaykh al-Albani rahimahullah has graded this hadith as hasan. See: [صحیح سنن أبي داود (مفصل) للألباني ، رقم : 383]

And the clothes should also be changed so that he feels a complete transformation and perceives himself as purified from the filth of disbelief. The dirt and filth will also be removed.

➋ The leaves of the lote tree (sidr) are for removing dirt and filth. Nowadays, soap can serve this purpose.

➌ According to Imam Ahmad rahimahullah, this ritual bath (ghusl) is obligatory (wajib), because the Prophet sallallahu alayhi wa sallam commanded it, and a command implies obligation. Moreover, disbelievers generally do not perform the ritual bath of major impurity (ghusl janabah), and even if they do, it is not done correctly, so they remain in a state of major impurity (janabah). Therefore, in order to attain purity, the ritual bath is obligatory. The apparent wording of the hadith also supports this view, so the position of obligation is the stronger one. And Allah knows best.