The Quran

قَدِ افتَرَينا عَلَى اللَّهِ كَذِبًا إِن عُدنا في مِلَّتِكُم بَعدَ إِذ نَجّانَا اللَّهُ مِنها ۚ وَما يَكونُ لَنا أَن نَعودَ فيها إِلّا أَن يَشاءَ اللَّهُ رَبُّنا ۚ وَسِعَ رَبُّنا كُلَّ شَيءٍ عِلمًا ۚ عَلَى اللَّهِ تَوَكَّلنا ۚ رَبَّنَا افتَح بَينَنا وَبَينَ قَومِنا بِالحَقِّ وَأَنتَ خَيرُ الفاتِحينَ

Muhammad Asad

We should be guilty of blaspheming against God¹ were we to return to your ways after God has saved us from them! It is not conceivable that we should return to them - unless God, our Sustainer, so wills.² All things does our Sustainer embrace within His knowledge; in God do we place our trust. O our Sustainer! Lay Thou open the truth between us and our people - for Thou art the best of all to lay open the truth !-³

Lit., "inventing a lie about God".
An expression of humility, and not of the idea that God might "will" them to blaspheme.
Or: "Thou art the best of all deciders" - since the verb fataha can also be rendered as "he decided". However, Shu'ayb's prayer could not have implied a request for God's "decision" (for there was no doubt in his mind as to who was right), and therefore the primary significance of iftah ("lay open") and fatih ("one who lays open", i.e., the truth) is preferable.