Imam Sadiq (a.s.) said to me (the narrator): “Husain! Allah will write a good deed for one who leaves his house to perform the Ziyarat of Imam Husain (a.s.) on foot and erase a sin from him for every step he takes. Till he arrives at the holy shrine, Allah will write him as one who has achieved salvation and as the successful one. After he completes the Ziyarat, Allah will write him as the successful one. When he decides to return, an angel will come to him and say: “Allah’s Messenger conveys his salutation on you and tells you that all your previous sins are forgiven. So continue good deeds.”
Imam Sadiq (a.s.) said: “Allah has some angels who are appointed at the grave of Imam Husain (a.s.). When a person decides to perform the Ziyarat of Imam Husain (a.s.), Allah gives his sins to these angels. With the first step that he takes, the angels will erase his sins and with any additional steps, they will multiply his good deeds until he becomes eligible for Paradise. Then they will surround him and sanctify him. They will call out to the angels of the heavens, “Sanctify the visitor of the beloved of the beloved of Allah.” When the visitor performs the Ghusl, Muhammad will call out to him: “Guests of Allah! Glad tiding to you, for you will join me in Paradise.” Then Amirul Momineen (a.s.) will call out, “I guarantee the fulfillment of your requests and to keep afflictions away from you in this life and hereafter.” Then the Holy Prophet (s.a.w.s.) will join them and stay to their right and left until they return to their kin.
Source: Book ‘Kamiluz Ziyaraat’ by Abil Qasim Ja’far bin Muhammad bin Musa (a.r.) (Ibne Quluwayh)