I beg to disagree. Please continue with your search.the only sources I have found pre-dating Ishaq are the following:
Aban b.`Uthman al-Bajkali
(640-718 C.E.), son of the caliph who wrote a book on maghazi which has not survived, nor has it been cited by Ibn Ishaq or al-Waqidi.
`Urwa b. al-Zubayr b. al-Awwam
(643-712 C.E.) the cousin of the Prophet and referred to the founder of Islamic history. There is doubt that he authored anything, but there are many traditiions that have been handed down in his name.
Urwah wrote many books but, fearing they might become sources of authority alongside the Qur'an, destroyed them the day of the Battle of al-Harrah. He later he regretted that, saying "I would rather have them in my possession than my family and property twice over."
Therefore it seems clear that there is no trace of any written works before Ishaq
Even the so-called books of Ibn Ishaq and Ibn Hisham have not survived. They do not exist.