Authentic hadith on the punishment of rapists:
Chapter regarding the punishment of the man who rapes the woman
Burayd Al-Ajali said: "Abu Ja'far was asked about a man who raped a woman". Abu Ja'far replied: "He is to be killed, whether he is muhsan or not"
Abi Abdullah said: "If a man forces himself (by rape) upon a woman, he is to be struck with a sword whether he dies from it or lives."
Zurarah said: I asked Abi Ja'far: "A man forces himself (by rape) upon a woman?" Imam (as) replied: "He is to be killed."
Source:
Kitab Al-Kafi, Volume 7, page 189
(Abi Ja'far (as) and Abi Abdullah (as) are the 5th and 6th Shia Imams, respectively. Shia Muslims consider them to have the same authority over Muslims as the Prophet Muhammed (s) did. Furthermore, Abi Ja'far also had influence over Sunni Islam. He was the teacher and mentor of Imam Malik and Imam Abu Hanifa.)
Fatwas by classical scholars on rape:
Scholar: Abu al-Qasim Ali ibn Husayn al-Sharif al-Murtadha (965 - 1044)
"...whoever forces himself upon a woman and rapes her, his neck is to be struck, whether he is muhsan or not.."
".....this act is the most obscene and disgusting in Islamic law and is worse than consensual fornication..."
Source: Al-Intisar, page 527
Scholar: Al-Shaykh Al-Mufid
Whoever forces himself upon a woman and rapes her, his neck is to be struck (with a sword), whether he is muhsan or not.
Source: Al Muqni'ah, page 778
(All sources provided are in Arabic)
The word 'Muhasan', refers to a man who is married, or has a concubine, or both.