The prophet (ṣallallāhu ʿalayhi wa-sallam) most certainly warned his ummah against deviation and fabrications within the religion. Whilst this is the case, many are adamant upon distorted principles and understandings of mūwāzanāt (counterbalancing the good and bad in preachers). This is especially so in our trying times, with groups such as the Ikhwān al-Muflisīn (The Bankrupt Brotherhood) spearheading the call to such misguidance, wa-Allāhu al-mustaʿān. Our pious salaf feared misguidance, and they used to warn against Ahl al-Bidʿah, and by Allāh they were not deceived by their beautiful speech nor their actions.

It is reported that Imām Layth b. Saʿd – Allāh have mercy on him – said, “Even if I saw a heretic (follower of bidʿah) walk on water, I would not accept him.” This was reported to Imām al-Shāfiʿī – Allāh have mercy on him, to which he replied, “He didn’t go far enough. Rather, if I saw one walk in the air, I would not accept him.” Al-Lālakāʿī, Sharḥ Uṣūl ʾIʿtiqād Ahl al-Sunnah Vol.1 p228; Ibn Baṭṭah, Al-Ibānah Al-Kubrá Vol2. P175; Ibn Al-Jawzī, Talbīs Iblīs p14 – with slightly variant wordings

Source: http://www.sayingsofthesalaf.net