Hashmi, 37, recently released a book on his son's battle with cancer, titled "Kiss of Life", which talks about the hardships he and his wife faced when their son was diagnosed with cancer at the age of four in 2014.
Bachchan, 73, who sent a special letter for the actor's son, called the little ones' strength unbeatable.
"Thank You for being such a brave father... And to Ayaan, what can one say... His strength and will is unbeatable," the "Wazir" actor tweeted.
Hashmi shared the special note from the Bollywood superstar on Twitter yesterday.
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