God says
O you who have attained to faith! Be ever steadfast in upholding justice, bearing witness to the truth for the sake of God, even though it is against your own selves or your parents or your kinsfolk. Whether the person concerned be rich or poor, God’s claim takes precedence over [the claim of] either of them. Do not then follow your own desires lest you swerve from justice: for if you distort (the truth), behold, God is indeed aware of all that you do.- Q.4: 135
* I.e., “do not allow the fact that a man is rich to prejudice you in his favour or against him, and do not, out of misplaced compassion fovour the poor man at the expense of the truth”
And on the Day of Judgment God will say: This is a day truthfulness shall benefit all who have been true to their word: theirs shall be the gardens through which running waters flow, therein to abide beyond the count of time; well pleased is God with them and well pleased are they with Him; this is the triumph supreme. -- Q.5: 119
Prophet said
1. “You shall speak truth because truth leads to piety and piety leads to paradise. A man continues to speak truth and adheres to the truth , till he is enrolled near God a great truthful man” – H: Bukhari and Muslim. N: Abdullah-b-Masud.
2. The Prophet was asked who among men was best? He replied, “Everyone pure of heart* and truthful of tongue” – H: Ibn Majah. N: Abdullah-b-‘Amr.
* According to the Prophet it is the pure heart that bears neither sin, nor transgression, nor deceit, nor envy (hasad).
