בשחר, צריך לאדם לומר: "מודה אני לפניך ה' אלהי ואלהי אבותי שהוצאתני מאפילה לאורה."במנחה, צריך אדם לומר: "מודה אני לפניך ה' אלהי ואלהי אבותי כשם שזכיתני לראות החמה במזרח, כך זכיתי לראות במערב."בערב, צריך לומר: "יהי רצון מלפניך ה' אלהי ואלהי אבותי כשם שהייתי באפילה והוצאתני לאורה, כך תוציאני מאפילה לאורהIn the morning prayer, it is necessary for a man to say: "I admit before you, God, my Lord and the Lord of my fathers, that you took me out from darkness to light."In the afternoon prayer, a man needs to say: "I admit before you, God, my Lord and the Lord of my fathers, just as you merited that I should see the sun in the east, such shall you cause me to merit to see in the west."In the evening prayer, he needs to say, "May it be the will from before you, God, my Lord and the Lord of my fathers, just as I was in darkness and you took me out to light, such shall you take me out from darkness to light."
I try to say these sayings every day, but am not sure why they are not codified להלכה (as Jewish law).
While not mandating that one needs to say these things, Rabbi Yisrael Meir Kagan wrote (in his באור הלכה, (in fact, the very first entry in that work(!)) on 1.1, s.v. שיהא הוא מעורר השחר) that
It is very proper to be careful to say before each of the three prayers a gratitude of the "May it be the will...", which is amplified in the Yerushalmi, chapter of tefillat hashahar.
I don't know why this is the furthest it's been "codified", but perhaps it's not so common because people didn't really learn the Yerushalmi all that much, or perhaps because no one thought to codify it. It's also possible that since there is a connection of this part of Rabbi Shmuel, son of Nahmani's statement with the first part that the three prayers each day are instituted because of the changing of the day. Perhaps because the final decision was that it was due to the offerings that prayer is done, his comment went by the wayside. Thus, Rabbi Kagan's position that it's very good to say them, refraining from using mandatory language, shows that, nevertheless, one ought to say them.