Tosefta sur Rois 1 8:28

I would have thought that [a person] should pray [Shmoneh Esreh] in a loud voice. [However] it explains by Chana, “and Chana was talking in her heart…” (I Samuel 1:13) I would have thought [a person] may pray all [three Shmoneh Esreh] in one shot. [However] it explains by David, “Evening, and morning, and afternoon…” (Psalms 55:18) Evening that is the Maariv prayer, morning that is the Shacharit prayer, and afternoon that is the Mincha prayer. I would have thought that [first a person] should ask for his [personal] needs and then he [should] pray [the established text of Shmoneh Esreh] and go away [from standing in front of God]. [However] it explains by Shlomo, “… to hear the song and the prayer …” (I Kings 8:28) Song that is the song,4 prayer that is the request. And so it says, “Sing before Hashem…” (Psalms 33:1)