שמואל א טו:כט פירוש: רש״י

But, the Eternal One of Yisroel neither lies. And if you will say, 'I will repent of my sin before Him,' it will no longer avail to take the kingship from the one to whom it was given, for the Holy One, Blessed is He, Who is the Eternal One of Yisroel, will not lie by not giving the good to whom He said to give it.