What would Muhammad, may God bless him and grant him peace, have read when Gabriel first commanded him with "Recite!"? Was it a message, a manuscript, or what?

The first word Gabriel said to the Prophet, peace and blessings be upon him, was "Iqra' (Read)." The intention behind it was either to alert him and make him vigilant for what was about to be revealed to him, or that the intention was for him to say this word because it is the first word of Surah Al-Alaq and the first revelation of the Quran.