Mishkat al-Masabih #181
Faith
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
وَعَن إِبْرَاهِيم بن ميسرَة قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مَنْ وَقَّرَ صَاحِبَ بِدْعَةٍ فَقَدْ أَعَانَ عَلَى هَدْمِ الْإِسْلَامِ» . رَوَاهُ الْبَيْهَقِيُّ فِي شُعَبِ الايمان مُرْسلا
Ibrahim b. Maisara reported God’s messenger as saying, “He who honours an innovator has assisted in undermining Islam.”
Baihaqi transmitted it in mursal form in Shu'ab al-iman.
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About Mishkat al-Masabih
This hadith is #181 from Mishkat al-Masabih, in the Book of Faith.