A white flag inscribed with the Shahada (in Maghrebi script).
This is supposedly a banner used by the Almoravid dynasty in Morocco (11th to 12th centuries).
This claim is apparently due to an entry at the "Flags of the World" website:

"The use of the flag in Morocco as a symbol of the state dates way back to the Almoravide dynasty (1062-1125 AD). Prior to this time, white silk banners were often carried in battle,sometimes with Koranic inscriptions written on them. The Almoravides institutionalized this practice. They gave one banner to every unit of 100 soldiers; the leaders always carried one inscribed: «There is no god but god, and Mohammad is His Prophet». The two following dynasties (the Merinides and the Saadiens) continued the use of the White flag as the symbol of the State." (Dov Gutterman, 17 Apr 1999)Note that no source is being given for this claim, the best we can do at this point is say that we heard it from Dov Gutterman in 1999.
Also note that the imlied claim that the modern form of the shahada was written on the flags. 

This is not only unsubstatiated but also implausible, as medieval sources tend to give a longer form of the shahada, lā ilāha illā-llāh waḥdahu lā šarīka lahu, muḥammadan ʿabduhu wa rasūluhu. It is completely unclear when the modern form is first recorded, let alone when it can first be shown to have been written on a flag (the earliest positive reference we have for this in the en-wiki's shahada article is the earliest 20th century, possibly (without explicit reference) traceable to flags used by Wahhab in the 18th century.
Own work by uploader - Morocco Historical Flags
Flad