Daniel Cormier insisted Khabib Nurmagomedov's retirement is "real" and the unbeaten UFC champion will not return to the Octagon.
Khabib announced his retirement after beating Justin Gaethje to defend his lightweight championship at UFC 254 in Abu Dhabi on Saturday.
The 32-year-old Khabib (29-0) was fighting for the first time since the death of his father and coach Abdulmanap Nurmagomedov in July.
Khabib was overcome by emotion as he burst into tears post-fight and former light heavyweight and heavyweight champion Cormier spoke about the Russian star's decision amid his mother's desire to stop.
"This is real," Cormier, a close friend of Khabib's, told ESPN. "Khabib doesn't lie. Khabib doesn't play 'I'm retiring and I'm coming back.'"
"When he said that, and he said, 'I give my word, and that's the only thing I have,' I was like, 'He's done'.
"He's never lied to me. Maybe he has messed around. We've had fun. We've played jokes. We've done a lot of things. But the one thing he doesn't do is lie.
"And I don't think he would lie to his mother, especially in the face of what they're dealing with as a family."
Khabib swiftly defeated interim champion Gaethje in a dominant display, winning by technical submission with a triangle chokehold in the second round.
"He told me when he was watching the interviews over the course of the week, he saw that Justin said that he would never tap," Cormier, who retired following his loss to Stipe Miocic at UFC 252 in August, said.
"He was going to do the armbar, but he had heard Justin saying all week that he would never tap, and he didn't want to hurt him in front of his parents. So he went to the triangle and just kind of put him to sleep."