Hadithcore
Mishkat al-Masabih, 189
hasan· Zubair `Aliza'i

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.

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

References1 variant
In-Book Reference
Book 1, Hadith 181