Reference: Fataawa Noor ‘alad-Darb – Question No.6256, Pages 94-95, Volume 12
Question: If a person who stopped praying repented, and then returned to [worshiping] his Lord – would he be required to verbalise the testimony of faith as well as having a wash (bath or shower)?
Response: If the person who does not pray repents to Allaah (‘Azza wa Jall), and then resumes his prayers, then by doing his [first] prayer he has become a Muslim. That is because whoever disbelieves by abandoning the prayer, then he returns to being a Muslim when he begins praying again. [Bearing in mind] whilst he will be praying, he will recite:
أَشْهَدُ أَنْ لَّا إِلَٰهَ إِلَّا الله وَ أَشْهَدُ أَنَّ مُحَمَّداً رَسُوْلُ الله
I testify that there is no deity truly worthy of worship except Allaah [alone], and I testify that Muhammad is the Messenger of Allaah
As for washing [- either taking a bath or shower], then that is based upon the obligation to do so when a disbeliever embraces Islaam.
So whoever is of the opinion that when a disbeliever embraces Islaam must have a wash, then when he resumes his prayer, it becomes incumbent upon him to have a wash.
And whoever is of the opinion that it is not obligatory to have a wash when he embraces Islaam, then he is not required to have a wash. However, there is no doubt that it is better to have a wash – just to be on the safe side.