What was reference of Newton which led to Gravitational law?W=mg(Surely it was not an apple!)

Isaac Newton began the investigations that led him to discover gravity after he saw an apple fall from a tree in his orchard. Actually, Gravity always existed, he was the first to postulate laws regarding how things are influenced by gravity-Laws 1,2 &3.
His law of universal gravitation was improved upon much later by scientists such as Albert Einstein and his theory of relativity.
His life story was published by William Stukeley in his biography, Memoirs of Sir Isaac Newtons life.

Newton articulated his three laws of motion plus the law of gravitation in his famous work "Philosophae Naturalis Principia Mathematica," (usually reffered to simokly as "Newton's Principia") in 1687.