It is reported that Imām Layth b. Sa’d – Allāh have mercy on him – said, “Even if I saw a heretic (follower of bid’ah) walk on water, I would not accept him.” This was reported to Imām Al-Shāfi’ī – 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-Lālakā'ī, Sharh Usūl ‘I’tiqād Ahl Al-Sunnah Vol.1 p228; Ibn Battah, Al-Ibānah Al-Kubrā Vol2. P175; Ibn Al-Jawzī, Talbīs Iblīs p14 – with slightly variant wordings.