ENOfficial
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.
Source: sunnah.com
Other translations
Arabic
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
sunnah.com