In Paul's first letter to the Corinthians Paul indicates that he prefers to pray both in (or with) the Spirit and in (or with) the mind.
τί οὖν ἐστίν; προσεύξομαι τῷ πνεύματι, προσεύξομαι δὲ καὶ τῷ νοΐ· ψαλῶ τῷ πνεύματι, ψαλῶ δὲ καὶ τῷ νοΐ· 1 Corinthians 14:15
In his letter to Ephesus Paul encourages them to "Pray always...in the Spirit."
διὰ πάσης προσευχῆς καὶ δεήσεως, προσευχόμενοι ἐν παντὶ καιρῷ ἐν πνεύματι, καὶ εἰς αὐτὸ ἀγρυπνοῦντες ἐν πάσῃ προσκαρτερήσει καὶ δεήσει περὶ πάντων τῶν ἁγίων Ephesians 6:18
The context of 1 Corinthians 14 suggests that Paul links praying in a tongue to praying in the Spirit based on his constant encouragement in the chapter to edify the church with words that can be understood. Yet, he also asks rhetorically (earlier in that letter), "Not all speak in tongues, do they?" Still, his instructions to Ephesus, "Pray always...in the Spirit" would prove difficult if praying in the Spirit could only be interpreted as praying in a tongue while not all speak in tongues. It would also seem to contradict Paul's preference that he prays at times "in the mind".
Are there other places in Scripture where translations for the phrase "προσεύξομαι τῷ πνεύματι" or simply "ἐν πνεύματι" provide clarity for these instructions?