Ashura

Imām Alī ibn Mūsā al-Rezā (A.S.) said:

"Muharram is a month in which bloodshed was considered unlawful by the pre-Islāmic pagan Arabs, but our blood was shed in this month. Our sanctity was violated and our children & women-folk were made captives. Our tents were set ablaze and whatever was found therein was looted. And they did not even honor the relation, which we share with the Prophet of Allāh (S). The day on which Imām Husayn (a.s.) was martyred has injured our eyes and our tears are constantly flowing since then. Our dear ones were dishonored on the plains of grief and trials (Karbo bāla) making way for sorrows and sufferings until Qiyāmah. Thus, sorrowful people should mourn over it (the martyrdom of Imām Husayn), for weeping upon it nullifies the major sins." 

 [References:  Nafasul Mahmum,  Chapter 3, Section 2, Tradition 15]

Imām Alī ibn Mūsā al-Rezā (a.s.) said:

“Whoever avoids attending to his worldly affairs on the tenth of Muharram, Allāh will fulfill all the wishes and desires of this world as well as the hereafter. Whoever considers this day to be a day of mourning, sorrow and weeping for himself, Allāh the Glorified will make the day of Qiyāmah to be a day of rejoicing for him and his eyes will be cooled in Paradise on account of us. And whoever considers the tenth of Muharram to be a day of prosperity and buys something for his house (considering it a good omen), then Allāh will not give him affluence in that thing. And on the day of Qiyāmah he will be made to arise along with Yazid, Ubaydullāh bin Ziyād and Umar ibn Sa'ad (may Allāh's curse be on all of them) and will be thrown into the lowest abyss of hell..”

[Reference: Nafasul Mahmum, Section 2,  Hadith 16]