He awoke, finding it difficult to breathe.
Leaping from bed, Youssef scrambled to make sure his 9-month-old twins were still alive.
Apparently unharmed, he passed them to his wife and told her to stay in the house.
Rushing outside to check on his parents next door, Youssef passed people staggering and falling in the street.
Youssef and many members of his extended family live on the northern edge of Khan Sheikhoun, a town in Syria's Idlib province. On Tuesday, airstrikes battered an area near their local bakery, meters from Youssef's home.
But it wasn't just any attack - Syrian President Bashar al-Assad has been accused of using chemical weapons in the strikes that rattled the rebel-held area while many were still sleeping.
Youssef arrived in his parents' house to find his two brothers dead. Panicked, he rushed back to his home to check on his wife and babies.
"There was foam on their mouths, there were convulsions. They had all been on the floor," Youssef told CNN on Wednesday, sobbing.
"My kids, Ahmad and Aya, and my wife... they were all martyred.
"My entire family's gone."
Youssef's wife and children were among the victims of what is feared to be the deadliest chemical attack in years in the country. Investigations are underway to determine who was responsible for the Tuesday morning attack that left at least 70 dead, according to activists. Syria's military has denied using chemical weapons and blamed rebels for the carnage.
Youssef says he collapsed and awoke a few hours later in a hospital bed to a shocking reality -- much of his extended family was dead.
He says about 25 of his relatives died that day in Khan Sheikhoun.
"My brothers, their children, and their cousins," Youssef says. "Around 25 members of my family, all martyred."
According to local reports on Tuesday, at least 19 members of the Youssef family were killed in the strike. His wife and twins were noted on the list of victims, which was circulated online.
On Wednesday, people in the northern Syrian town gathered to bury the dead.
A picture of Youssef cradling his children, both wrapped in white sheets, went viral on social media. Youssef was photographed sitting at the edge of a burial site, holding his twins. Footage, filmed later, shows him kneeling by their graves and weeping.
"I am crying, but these are tears of joy," Youssef says, explaining that his children are now with God, and that is better than being in Syria.
"My kids aren't the first kids to die," he adds, breaking down in tears.
(Pics : Dailymail)
(Except for the headline, this story, originally published by CNN.comhas not been edited by SLM staff)