Brewer's: Alnaschar Dream (An)
Counting your chickens before they are hatched. Alnaschar, the barber's fifth brother, invested all his money in a basket of glass-ware, on which he was to make a certain profit. The profit, being
invested, was to make more, and this was to go on till he grew rich enough to marry the vizier's daughter. Being angry with his imaginary wife he gave a kick, overturned his basket, and broke all his wares.
“To indulge in Alnaschar-like dreams of compound interest ad infinitum.” —The Times.