Accept the truth, whoever it comes from
It is reported that a man came to ‘Abdullâh b. Mas’ûd – Allâh be pleased with him – and asked, “Teach me some comprehensive and beneficial words.” He replied, “Worship Allâh and do not associate any partners with Him, and be with the Qur’ân wherever it is. And whoever comes to you with some truth – whether he is young or old, even if he is hated by you – then accept [that truth]. And whoever comes lying to you – even if he is beloved and close – then reject it from him.”
Abû Bakr Al-Kharâ`itî, Masâwî Al-Akhlâq wa Madhmûmihâ p72.