A doubtful friend is worse than a certain enemy. Let a man be one thing or the other, and we then know how to meet him. 

Aesop (620-560 BC), known only for the genre of fables ascribed to him, was by tradition a slave  who was a contemporary of Croesus and Peisistratus in the mid-sixth century BC in ancient Greece.

Treat your friend as if he might become an enemy.

Publilius Syrus

He who has a thousand friends has not a friend to spare,
And he who has one enemy will meet him everywhere.

Ali ibn-Abi-Talib (599 or 600-661) Was the cousin and son-in-law of the Islamic prophet Muhammad, who ruled over the Rashidun empire from 656 to 661

It is easier to forgive an enemy than to forgive a friend.

William Blake (1757 – 1827) was an English poet, painter, and printmaker.

He who has a thousand friends has not a friend to spare, and he who has one enemy will meet him everywhere.

Ralph Waldo Emerson

It is easy enough to be friendly to one’s friends. But to be friend the one who regards himself as your enemy is the quintessence of true religion. The other is mere business.

Mohandas Gandhi (1869 -1948 ) A major political and spiritual leader of India and the Indian independence movement.

Love is the only force capable of transforming an enemy into friend.

Martin Luther King, Jr

The best way to destroy an enemy is to make him a friend.

Abraham Lincoln