1 Corinthians 14:14

Public prayer is in view here, as it is in this whole chapter (1 Corinthians 14:16), but some may have been praying in tongues privately as well. While praying in a tongue might give the person doing so a certain sense of exultation in his spirit, his mind would not benefit. He would not know what he was saying without interpretation. The "spirit" (Gr. pneuma) seems to refer to that part of the person that exercises this spiritual gift. It is separate from the mind obviously (cf. 1 Corinthians 14:4). The person’s spirit prays as the Holy Spirit gives him or her utterance.