Mishkat al-Masabih #400
Purification
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
وَقَالَ ابْنُ عَبَّاسٍ: إِنَّمَا الْمَاءُ مِنَ الْمَاءِ فِي الِاحْتِلَامِ. رَوَاهُ التِّرْمِذِيُّ وَلَمْ أَجِدْهُ فِي الصَّحِيحَيْنِ
Ibn ‘Abbas said that "Water is necessary only when there is an emission”
applies to a dream.
Tirmidhi transmitted it, but I have not found it in the two Sahihs.
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About Mishkat al-Masabih
This hadith is #400 from Mishkat al-Masabih, in the Book of Purification.