Imam al-Shafi'i (may Allah have mercy on him) said: "Among these (forms of testimony) is that which the witness has (himself) observed, so he testifies on the basis of observation."
The Shaykh (Imam al-Bayhaqi, may Allah have mercy on him) said: "What is meant by this are the actions that you observe and then testify to on the basis of observation."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / Q20581