This is the second book of the Mishneh Torah of the Rambam, Rabbi Moshe ben Maimon. Laws of Tefillah.
This chapter explains that anyone who prayed but lacked concentration must pray again. This includes someone who made a mistake in the first or last three blessings, or a messenger from the congregation while praying aloud. It also includes a messenger from the congregation who doesn't know where they went wrong or who made a mistake in the blessing against heretics; someone who says they will not go down to the Ark or who is unsure whether they have already prayed; someone who is unsure whether they have already prayed or recited the weekly Shabbat prayer; someone who finished the weekly prayer thinking it was the additional one (Musaf); someone who did not mention rain or dew during the winter days or make a request in the New Year blessing; someone who did not mention the Rosh Chodesh (beginning of the month) prayer or finished the third blessing, "The Holy King," during the Ten Days of Repentance; someone who did not mention Havdalah (separation) in the blessing over understanding or who forgot "for miracles" or "answer us." Anyone who did not say the evening prayer on Friday or Saturday. Anyone who says two prayers one after the other.