Matthew 9:33

And when the demon was cast out, the dumb man spake: and the multitudes marveled, saying, It was never so seen in Israel. But the Pharisees said, By the prince of the demons casteth he out demons.

See under Matthew 12:24ff for more on the prince of demons. The Pharisees were absolutely unwilling to admit any good thing in Jesus. When they were unable to deny his wonderful deeds, they questioned the source of his power. In addition to accusing Christ of being in league with the devil, they made a big issue of the cures wrought on the sabbath day; and, it may be assumed, they denied, whenever practical, that any good deed had been done. There is an implicit admission of this in their words, "A notable miracle hath been wrought through them ... and we cannot deny it" (Acts 4:16). This is very nearly the equivalent of their saying that they would have denied the miracle of the apostles performed at the Beautiful Gate of the temple, if they could have done so with any prospect of being believed.