Astro Boy

Toby Tenma is a teenager who lives in the futuristic city-state Metro City, which floats above the polluted "Surface". Toby's father, Dr. Tenma, is a famous roboticist and head of the Ministry of Science, but has a distant relationship with his son. Dr. Tenma meets the city's obstructive leader President Stone to demonstrate a new defensive robot called the Peacekeeper. To power it, Tenma's friend Dr. Elefun unveils the Blue and Red Core, two energy spheres which have opposing positive and negative energy. Stone has the destructive Red Core placed into the Peacekeeper, causing it to go berserk, leading to Toby being disintegrated after sneaking into the room.
A distraught Tenma secretly constructs a robotic replica of Toby, complete with his memories, but has built-in defenses to protect him and is powered by the Blue Core. Toby's replica is awakened, believing nothing has happened. While he has Toby's mind, his personality is different in Tenma's eyes, when in actuality it is quite similar. Toby discovers his robotic capabilities including rocket-powered flight and the ability to understand other robots. Toby flies home but learns from Tenma of his origins and is rejected by him, flying away much to the sadness of Dr. Elefun. Stone, desperate to win a re-election, has Toby pursued by his guards but the battle leads to Toby tumbling off the city edge onto the Surface. Tenma and Elefun are arrested.
Toby awakens in an enormous junkyard, created from the redundant robots dumped by Metro City. He is found by a group of human children, illiterate but smart Zane, twins Sludge and Widget, and the oldest Cora who has a grudge against Metro City. They accompanied by a dog-like waste disposal robot named Trashcan. Toby also meets the members of the Robot Revolutionary Front (RRF), Sparx, Robotsky, and Mike the Fridge, who wish to free robots from mankind's control but are very inept and bound by the Laws of Robotics. However, they give Toby a new name, calling him "Astro". Astro departs with the kids, finding people still live on the Surface. He is taken in by robot repairman Hamegg, who also runs a robot fighting ring. The next day, Astro comes across an offline construction robot Zog, whom he revives with the Blue Core's energy. Hamegg accidentally scans Astro, realizing he is a robot, and paralyzes him the next day to participate in the fighting ring.
Astro defeats Hamegg's fighters until Zog is deployed, but the two robots refuse to fight one another. Hamegg assaults them with an electrical blaster, only for Zog to attempt to harm him, immune from the Laws of Robotics, but Astro saves Hamegg. President Stone's flagship arrives and Astro is taken back to Metro City. Reuniting with Tenma and Elefun, Astro agrees to be shut down, apologizing to his father for not being Toby. Realizing Astro is still his son, Dr. Tenma reactivates him and lets him escape. Stone loads the Red Core into the Peacekeeper, only for it to absorb him and take on his personality. The Peacekeeper absorbs weapons and buildings, growing in size until it's the height of skyscrapers. It then goes on a rampage across Metro City, prompting Astro to battle it. During the fight, Metro City's systems shut down, causing it to tumble to the ground. Astro uses his superhuman strength to help it land safely.
The Peacekeeper grabs Astro, but the connection of their cores causes them both pain. Dr. Tenma finds Astro and informs him that if the two cores reunite, they will be destroyed. When Astro's friends are captured, he flies into the Red Core, destroying himself and the Peacekeeper. Stone survives but is arrested for his crimes. Dr. Elefun and the children find Astro's body, discovering the Blue Core is dead. Zog appears and revives Astro using the Blue Core energy given to him. Astro reunites with his loved ones, making peace with his father. A cycloptic extraterrestrial attacks the city, Astro leaping into action as the film ends.

Question: Who tells Astro that if the cores reunite, they will be destroyed?
Answer: Dr. Tenma