Take a look at Matthew 6:5 - 7
5. And when thou prayest, thou shalt not be as the hypocrites: for they love to pray standing in the synagogues and in the corners of the streets, that they may be seen of men. Verily I say unto you, They have their reward.Pretty clear, isn't it? Public prayer is considered to be a form of hypocrisy while private prayer is valued.
6. But thou, when thou prayest, enter into thy closet, and when thou hast shut thy door, pray to thy Father which is in secret; and thy Father which seeth in secret shall reward thee openly.
7. But when ye pray, use not vain repetitions, as the heathen: for they think that they shall be heard for their much speaking.
For more, see my 2007 post on public prayer.