As discussed under Alma 30:37, Oliver Cowdery seems to have mistakenly written sayest here in Alma 45:2 rather than sayeth (his typical spelling of saith). When Oliver took down the text in 𝓞, he was apparently influenced by the following believest (which is correct since its subject is thou). Oliver made the same error, replacing the ending -eth with -est, in Alma 30:37 (for other examples of this kind of anticipatory error, see under 1 Nephi 11:2). Here in Alma 45:2, Oliver changed sayest to sayeth when he copied the text from 𝓞 into 𝓟. The 1830 compositor set sayeth as saith.
In the original text, this sentence began with the past-tense came but was followed by the historical present-tense saith. Such mixture in tense was common in the original Book of Mormon text (as well as in the King James Bible); see the many examples listed under Jacob 5:75. In the 1837 edition, this instance of the historical present-tense saith was edited to the past-tense said. The critical text will restore saith, the probable reading of the original text here in Alma 45:2.
Summary: Accept in Alma 45:2 the historical present-tense form saith, Oliver Cowdery’s emendation in 𝓟 (spelled as sayeth); the reading in 𝓞, sayest, is an error that most likely resulted from the following nearby occurrence of believest.