We have already posted few quotes from shia books on this.
However, just another one.
Muhammad Hasan al-Istahbanati narrated in his book “Noor al Ayn” (p 52):
1) From Zayd Shahham: I said to Abu Abdullah (alaihi salam): What for a person who visits Hussayn? He answered: He like visited Allah on his Throne.
2) From Bashir Dakhan from Abu Abdullah (alaihi salam), which said to him in one hadith: O Basheer, who visited grave of Hussayn, acknowledging his rights, like visited Allah on his Throne.