Truthfulness is approached here from several angles: the Quran and the Messenger ﷺ confirm what came before them, and none is truer in speech than Allah. The verses mention prophets of perfect truthfulness, genuine truthfulness made of faith and deeds, the reward of the people of truth, the command to be with the truthful, and Mary, daughter of ʿImran, described as truthful.
Verses gathered from a topical index of the Quran, in Mushaf order; every verse is quoted in full.
And when a messenger from Allāh came to them confirming that which was with them, a party of those who had been given the Scripture threw the Scripture of Allāh [i.e., the Torah] behind their backs as if they did not know [what it contained].
And [recall, O People of the Scripture], when Allāh took the covenant of the prophets, [saying], "Whatever I give you of the Scripture and wisdom and then there comes to you a messenger confirming what is with you, you [must] believe in him and support him." [Allāh] said, "Have you acknowledged and taken upon that My commitment?" They said, "We have acknowledged it." He said, "Then bear witness, and I am with you among the witnesses."