Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both), concerning the saying of Allah Almighty: "Testify for Allah with justice, even if it is against yourselves or against your parents and close relatives" [Surah al-Nisa': 135], says: Even if it is against your fathers or your sons; and do not favour a rich man because of his wealth, nor show mercy to a poor man because of his poverty (so that you fail to give true testimony). The saying of Allah Almighty: "If he is rich or poor, Allah is closer to them both (and more caring of them)" [Surah al-Nisa': 135]. The saying of Allah Almighty: "So do not follow your desires, lest you fail to act justly" [Surah al-Nisa': 135], meaning: do not abandon the truth and commit injustice.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20590