It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that a man who used to give testimony (without verification) was mentioned in the presence of the Messenger of Allah (peace and blessings be upon him), and he said to me: "O Ibn Abbas, testify only to that which is as clear to you as the light of this sun." And the Messenger of Allah (peace and blessings be upon him) pointed with his blessed hand towards the sun. This hadith has a sahih (authentic) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.