I've never heard about this before. Amazing if true.
In 628 CE, a delegation from St. Catherine’s Monastery came to Prophet Muhammed and requested his protection.Source: Middle East Online
He responded by granting them a charter of rights, which I reproduce below in its entirety. St. Catherine’s Monastery is located at the foot of Mt. Sinai and is the world’s oldest monastery.
It possesses a huge collection of Christian manuscripts, second only to the Vatican, and is a world heritage site. It also boasts the oldest collection of Christian icons. It is a treasure house of Christian history that has remained safe for 1400 years under Muslim protection.
The Promise to St. Catherine:
"This is a message from Muhammad ibn Abdullah, as a covenant to those who adopt Christianity, near and far, we are with them.
Verily I, the servants, the helpers, and my followers defend them, because Christians are my citizens; and by God! I hold out against anything that displeases them.
No compulsion is to be on them. Neither are their judges to be removed from their jobs nor their monks from their monasteries. No one is to destroy a house of their religion, to damage it, or to carry anything from it to the Muslims' houses.
Should anyone take any of these, he would spoil God's covenant and disobey His Prophet. Verily, they are my allies and have my secure charter against all that they hate.
No one is to force them to travel or to oblige them to fight. The Muslims are to fight for them. If a female Christian is married to a Muslim, it is not to take place without her approval. She is not to be prevented from visiting her church to pray. Their churches are to be respected. They are neither to be prevented from repairing them nor the sacredness of their covenants.
No one of the nation (Muslims) is to disobey the covenant till the Last Day (end of the world)."