Mark 2:16

And the scribes of the Pharisees, when they saw that he was eating with the sinners and publicans, said unto his disciples, How is it that he eateth and drinketh with publicans and sinners?

A feast of the size indicated here was probably held in the courtyard of the house, leaving passers-by an opportunity to observe all that was done. For a long while the Pharisees had opposed Jesus; and their cunning and craft were evident in their maneuvers here to open a wedge between the Lord and his disciples.

And when Jesus heard it ... Christ, of course, knew all that was said or even thought by those in his presence; but this seems to indicate that Jesus' disciples immediately told him what the Pharisees had said, or that he perhaps overheard them. At any rate, he promptly answered the objection. See next verse.