Midrash sur Les Psaumes 62:13

(16) (Ib. b) (Ex. 34, 6), And the Lord passed by before him and proclaimed. R. Jochanan said: "Had this passage not been written, it would have been impossible to think of it, for it teaches us that the Holy One, praised be He! wrapped Himself, as does a deputy of the congregation, while pointing out to Moses the regular order of prayer, and said to him: 'Whenever Israel sins, let them do [pray to Me], after this manner, and I shall pardon their sins.'" (Ib.) The Lord, the Lord; i.e., I am the same God before a man sins as I am after he sins and does repentance; a God, merciful and gracious. R. Juda said: "This means a solemn assurance was given that the invocation of the thirteen divine attributes will never be without effect, as it is said (Ib. 10) Behold I make a covenant." Ilpha pointed out a similar contradiction: "It is written (Ex. 34, 6) abundant in goodness; and in addition it is written, and truth. How can both harmonize? In the beginning [His attribute] is truth, but at the end it is goodness." R. Elazar points out a further contradiction: "It is written (Ps. 62, 18) Unto thee, O Lord, belongeth mercy; and again. Thou renderest to every man according to his work; i.e., in the beginning He rewards every man according to his works, but in the end He is merciful." R. Huna points out a contradiction: it is written (Ib. 145, 17) The Lord is just in all His ways; and it is also written (Ib.) and kind in all His deeds; i.e., in the beginning He is only just, but in the end He is kind."