All perfect praise be to Allah The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
By analogy to the two voluntary Raka's offered after making ablution, performing two Raka's after Ghussl from a major impurity, such as Janabah(washing sexual impurities), menstruation and post-natal bleeding is an act of Sunnah. This is because the Prophet (PBUH) said (What means): "Bilal, narrate to me which act at the time of morning prayer you did in Islam for which you hope to receive good reward, for I heard during the night the sound of your steps before me in Paradise. Bilal said: I did not do any act in Islam for which I hope to get any benefit but this that when I perform complete ablution during the night or day I observe prayer with that purification what Allah has ordained for me to pray. [Related by Bukhari].
Interpreting "When I perform complete ablution," it is mention in {Omdat Al-Qareea' Sharih Sahih Al-Bukhari} that it includes Ghussl. Upon pointing out voluntary prayers,
Al-Imam Ar-Ramly (May Allah have mercy on his soul) said: "They (Al-Ramli and Al-Balqini) are of the view that performing two Raka's after making ablution-Al-Balqini added "and this applies also on Ghussl and Tyamum (Purification with sand/dust)- is an act of Sunnah." In his book [Al-Hashyah], Al-Ramli made the following comment:" Performing Two voluntary Raka's after making any kind of Ghussl is recommended." Kindly refer to [Mughni Al-Muhtaj].
This is also because performing ablution from a minor impurity goes under Ghussl from a major impurity.
In addition, it is from Sunnah that the one making the Ghusl performs ablution, before or after it (Ghusl), and whether he makes ablution or not, it is recommended that he offers two Rak`as after the Ghusl.
Nevertheless, no sound narration has ever mentioned the reward of performing two voluntary prayers after making Ghussl from menstruation. Although people used to narrate the following supposing that it is a Prophetic Hadith: "If a woman performed Ghussl from menstruation and offered two voluntary prayers in which she recited Al-Fatiha once and Al-Ikhlas thrice in each (Raka'), then all of her sins will be forgiven, and won't be counted till the next period. She also receives the reward of sixty martyrs….etc."- is a fabricated one; thus it isn`t permissible to offer any act of worship based on it nor to believe in any of it. In conclusion, a woman should perform two voluntary prayers after making Ghussl from menstruation as indicated by the narration of Bilal Ibn Rabah (May Allah be pleased with him). And Allah Knows Best.
