Were the Prophetic hadiths transmitted orally or in writing, and what does the difference in the reading of some words in hadiths, such as Abu Bakra's hadith "May Allah increase you in eagerness, and do not return," indicate about the method of hadith transmission?

The correct way to narrate the hadith: "wa la ta'ud" is with the opening of the ta and the ضم (damma) of the 'ain, as other narrations did not include it. The memorizers (huffaz) verify the correct narrations. As for the method of transmitting the Sunnah, the Companions received the hadiths from the Prophet verbally, and then they were written down at a later time. Writing from the shaykhs or reading to them is considered the highest degree of transmission, because receiving from the mouths of scholars safeguards against textual corruption.