Tawus narrates from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that a man who gives testimony was mentioned in the presence of the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) said: "As for you, O Ibn Abbas, do not testify to anything except a matter that is as clear to you as this sun," and he (peace and blessings be upon him) pointed with his hand towards the sun.
Al-Humaydi criticised this Muhammad bin Sulayman bin Mismul, and it has not been narrated through any reliable chain; and Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20579