Prayer is an act of the reason

"Prayer is an act of reason directing [(rationis . . . applicantis)] the ardent desire of the will at [(ad)] Him who is not under our power, but above us, namely God."

"Oratio rationis est actus, applicantis desiderium voluntatis ad eum qui non est sub potestate nostra, sed supra nos, scilicet Deus."

     St. Thomas Aquinas, Super Sent. IV.15.4.1.1.co.  Cf. also "Prayer is the act of reason by which one entreats a superior [(Oratio est actus rationis, per quem aliquis superiorem deprecatur)]" (ST II-II.83.10.co).

     I was put onto these by Christoph J. Amor, "'Asking makes a difference . . .':  Das Bittgebet bei Thomas Aquinas in der neueren Diskussion," Neue Zeitschrift für Systematische Theologie 50 (2008):  42 (37-61).  Cf. Super Sent. IV.15.4.1.1.ad 2 and ad 4; IV.15.4.1.2.co; IV.15.4.6.2.arg 2; and IV.15.4.6.3.co.  And ST II-II.83.1.co and ad 2 (to restrict this to the nouns oratio (#5730), ratio (#68404), and actus (#01503) only, in proximity).