The volumes of Risaala-e-nouin aren't directly authored by Imam Khomeini, Abdul-Karim Bi Azar Shirazi has selected and translated some parts of Imam Khomeini’s book “Tahrir al-Wasilah” and commentated on and explained those parts, altogether making up the book “Risaala-e-nouin”.

Regarding Imams Hasan and Husein (as) praying behind Marwan, there is a hadith which reads:

“نَوَادِرُ الرَّاوَنْدِیِّ، بِإِسْنَادِهِ عَنْ مُوسَى بْنِ جَعْفَرٍ عَنْ أَبِیهِ ع قَالَ کَانَ الْحَسَنُ وَ الْحُسَیْنُ ع یُصَلِّیَانِ خَلْفَ مَرْوَانَ بْنِ الْحَکَمِ فَقَالُوا لِأَحَدِهِمَا مَا کَانَ أَبُوکَ یُصَلِّی إِذَا رَجَعَ إِلَى الْبَیْتِ فَقَالَ لَا وَ اللَّهِ مَا کَانَ یَزِیدُ عَلَى صَلَاة”[1]

As for the authenticity of this hadith and whether it can be used to issue a fatwa or not, it is something that needs to be discussed elsewhere.

What is important is that in “Tahrir al-Wasilah”, there is no direct mention of Marwan or the abovementioned hadith. We also don’t have access to the electronic version of Risaala-e-Nouin to be able to give a definite answer in this regard; all we can say is that if this matter has indeed been mentioned in this book, it definitely has to do with the aforementioned hadith.