by little and little
(dated) A small amount at a time; little by little.
He that avoideth not small faults, by little and little falleth into greater. Thomas à Kempis
Deviation from either truth or duty is a downward path, and none can say where the descent will end. – 'He that despiseth small things shall fall by little and little. Tryon Edwards
Riches amassed in haste will diminish, but those collected by little and little will multiply. Johann Wolfgang von Goethe