is it true that if you say the fajr and ishaa prayer every day, your entire sins will be erased?(except for shirk of course)
I have never come across such Hadith. But it is mentioned in Saheeh Muslim, Kitabussalaah that whoever performs Ishaa Salaah in Jamaat, it is as if he performs up to midnight and whoever performs Farj Salaah in Jamaat, it is as if he performs the whole night.
In another Hadith of Sahihain it is mentioned that the Nabee of Allaah asked the Sahabah whether any filthiness would remain on the body of a person who got a river at his door and showered five times daily. The Sahabah replied, “Nothing of his dirt will remain”. Thereupon the Nabee of Allaah said, “That is the example of five daily Salaah by which Allaah wipes out sins.
Here it should be carried in mind that wherever in the Qur’aan and the Ahadith it comes that sins will be wiped out by righteous deeds, it refers to minor sins. Major sins won’t be washed out without sincere Tawbah.