Roland Bainton, Here I stand: a life of Martin Luther (New York and Nashville: Abingdon Press, 1950), 112, ll. 17-18 (chap. 6, "The Saxon Hus," under "The Leipzig debate").
Bainton's endnotes make it clear that he is working with the Walch edition of Luther's Schriften, though he does not give his source for these words in particular, but only l. 13 on the one hand, and l. 24 on the other. Clyde Leonard Manschreck, writing in 1958 (Melanchthon: the quiet reformer, pp. 46 and 323), uses Bainton's very words, does not place them within quotation marks, and cites CR 1, 91 (I am indebted to Dan Graves for reminding me to follow up on that use of the aphorism). And Gregory B. Graybill, writing in 2015 (The Honeycomb Scroll: Philipp Melanchthon at the Dawn of the Reformation, p. 181), uses Bainton's very words, does not place them within quotation marks, and cites MBW 59 =CR 1, 91, both of which would 1) match Walch, vol. 15, no. 394 (col. 1219 in this printing of -1910), i.e. the letter of Melanchthon to Oecolampadius dated 21 July 1519 and 2) fall right into the middle of the other Walch passages cited by Bainton on both sides of ll. 17-18. Yet Melanchthon isn't (at that very point, at least) nearly so aphoristic. Indeed, the word "truth" does not occur, whether in the original Latin or in Walch's German translation:
First Eck pleaded [(caussatus est)] with those who had been designated prefects of the disputation by the most illustrious prince George, the duke of Saxony, patron of humane learning, contrary to what had been agreed ([namely] that [the disputation] seem/be seen to be [governed] by the law of disputants), [that] it not be written down [(ne dictaretur)], that the inflammatory force/violence of those who, about to fight [(pugnaturi, FAP-NMP)], dispute in words [might not] gradually cease to boil [(defervere)] by reason of the pause[s] for writing [(per stili moram)]; [since] in the attack spirits [might] rise, [but] in the delaying [(contando < cunctando < cunctor? Walch: Zaudern) might] give way. For that reason I do not know whether [that] could be seen from [the perspective of] 'theological simplicity', given that [(ubi, when)] nothing must stand out to such a degree that everything seems [to be] said as an attack or impetuously or in an intemperate spirit [(nihil tam praestandum est, quam ut ne quid impetu, ne quid temere, ne quid immodico animo dictum videatur)]. And that in studies of literature [(studiis literarum)] and above all in the business of piety [(negocio pietatis)] I think nothing either more important or more wholesome in the close combat/encounter [(congressu familiari)] of the learned and good [than] when [(ubi)] opinion/proposition [(sententia)] is compared with opinion/proposition by gentle and tranquil and minimally pertinacious spirits, [and] when [(ubi)] it is neither disgraceful to be conquered nor praiseworthy to conquer; so that I consider scarcely anything more pernicious than these popular debates in which [(ubi)] the concern for victory cannot but raise a clamor against [those on the opposite side], however well-intentioned [they be (ubi non potest quantumvis bonis non obstrepere victoriae cura)].MBW 59 =CR 1, 91, trans. Steve Perisho, with input from Dr. Owen Ewald.
Unless someone can point me to some other report on the preliminaries of the Leipzig Disputation (but I have searched those gathered in vol. 15 of the Walch edition used by Bainton, Neue revidirte Stereotypausgabe (St. Louis, MO: Concordia Publishing House, 1899), cols. (802) 844-1259 (1377), for occurrences of "Wahrheit" without turning up a match that stood out to me), the phraseology should therefore be attributed to Bainton, not Melanchthon, despite the fact that Bainton employed quotation marks. Indeed, Graybill wrote on 9 August 2016 to confirm that he "chose not to use quotation marks around that specific phrase because [he] did not find a direct quotation to that effect", and that though "[Bainton's] use of quotation marks is curious," "it's more likely [his] summary rather than Melanchthon's quotation."
The editor of MBW 59 (in MBW T1) references, at this point (following Melanchthon himself), in addition to Gregory of Nazianzus' De moderatione in disputationibus servanda (MPG 36, cols. 174-212), and the translations of that by Melanchthon and Oecolampadius, two of the adages discussed at length by Erasmus, namely I.iii.7 ("Quot homines, tot sententiae", "So many men, so many opinions", CWE 31, pp. 240-241) and IV.i.1 ("Dulce bellum inexpertis", "War is a treat for those who have not tried it", CWE 35, pp. 399-440). But neither of them seem to lie directly behind the Bainton rendition.
This would not be the only point at which Bainton interjected his own personality into Here I stand.