A Bronx couple and their 5-year-old son were found stabbed to death inside their apartment building early Sunday, law enforcement sources and neighbors said.

Police responding to a 911 call at 674 East 136th Street in Mott Haven first discovered 38-year-old Jonathan Rivera, with stab wounds to the chest in the hallway of the first floor of the apartment building, at around 6:40 a.m., cops and the sources said.

As they investigated, the officers looked into the window of the nearby apartment unit — and broke inside after spotting the child, identified by police as Kayden Rivera, according to the sources.

The 33-year-old woman, Hanoi Peralta, was also discovered inside the apartment with fatal stab wounds in what the sources said was being investigated as a triple homicide.

The man, woman and child were pronounced dead at the scene by EMS, police said.

No suspects were publicly identified, and police said the investigation was ongoing.

Chilling security video from Neighborhood Grocery, a bodega just steps away from the front door of the building, shows Peralta casually shopping on Saturday afternoon — just hours before she was slain.

Storeowner Dahan Ali said the family frequented the store. 

“I feel so terrible,” Ali said. “So sad. She was here yesterday at 3 or 4. She was buying some things like normal. Very nice woman. Her husband, her son, all very nice people. I see them every day for the last year.

“Every morning she takes her son to school. Very beautiful boy, such a nice boy,” he added. “Her husband comes in every day for coffee.”

Other neighbors said all three had recently moved into the unit and mostly kept to themselves.

Akakpo Kodjogan, 62, who has been in the building since 2007, said the apartment they lived in had previously belonged to the building’s superintendent. 

“New people in there and now I’m not sure who is living there. This building used to be very good. Very safe. No problems,” Kodjogan said. “Now the people who are coming in, no one knows who they are.”

Another neighbor, a 41-year-old man who asked to not use his name, confirmed it was just the three people living in the super’s old apartment. 

“They’re quiet people… I don’t really know them that well,” he said, noting that he would say “hi” to them if he passed them in the hallway.

“I was home all night and I didn’t hear anything. I didn’t hear no yelling, no screaming. I don’t know what happened but they didn’t make any noise,” he added.

Medey Mercado, 55, who has lived on the fifth floor of the building for 10 years, described seeing the man’s butchered body inside the building earlier that day. 

“This morning there was a man lying in the lobby. He was bloody and cut up!” she said.

An autopsy was being conducted to determine the cause of death.

“I can’t believe it,” Ali, the storeowner, told The Post. “I am scared. When three people in the building get killed, yes, yes, I am scared. I don’t feel safe.”