What is the requisite basis for valid testimony regarding a matter?

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Testimony is not valid except concerning that which is known through certain means. This is evidenced by the saying of Allah: Except for one who testifies to the truth and they know it (Quran 28:86), and by the saying: And do not pursue that of which you have no knowledge. Indeed, the hearing, the sight, and the heart—each of those is going to be questioned (Quran 17:36). The latter verse singles out these three because knowledge resides in the heart, which relies upon hearing and sight, and the means of perceiving testimony are seeing and hearing. It is narrated from Ibn Abbas that when the Messenger of Allah (PBUH) was asked about testimony, he said: Do you see the sun? He replied: Yes. He said: Testify upon the like of it, or leave it. This was reported by Al-Khall in his 'Al-Jami'.