You already have a reputable source for the claim in the CNN article that you provided.
This claim has been repeated by different sources as well:
In an interview with Baladna TV, a private Palestinian news outlet that broadcasts via Facebook, senior Hamas official Salah Bardawil said 50 out of the nearly 60 protesters killed Monday were Hamas members, with the others being "from the people."
Bardawil did not elaborate on the nature of their membership in the group and his claim could not be independently verified. It was unclear if the protesters he was referring to were militants or civilian supporters of the Islamic group, which rules Gaza and opposes Israel's existence.
A Hamas official has claimed that 50 members of the Palestinian terror group were among those killed by Israeli fire during Monday’s mass protests at the Gaza border.
Hamas leader Salah Bardawil said in a televised interview that of the “62 people martyred, 50 were Hamas.”
The Hamas leader’s claim of 50 members brings the total of terrorists killed on Monday up to 53. The Iran-backed Palestinian Islamic Jihad named three of its members on Monday who were killed in the riots.
As well as non-English sources (used Google translate for translation):
"Sixty-two people were killed" on Monday and Tuesday, said Salah al-Bardaouil, questioned about the criticism that Hamas was benefiting from the mobilization. "Fifty of the martyrs (dead) were Hamas, how could Hamas reap the benefits (of the movement) while paying such a high price," he asked.
A spokesman for Hamas says that 50 of the 60 who were killed Monday and Tuesday were members of Hamas.
In addition to the reports by the media, the original interview is available and was translated by Memri Tv. The full transcript is:
Interviewer: "Many people are saying that the children... I'm telling you what people are saying. It's not that I believe this. People are saying that children are dying and that Hamas is reaping the fruits."
Salah Al-Bardawil: "In the last round, there were 62 martyrs."
Salah Al-Bardawil: "50 of the martyrs were from Hamas, and the other 12 were regular people. So how can anyone claim that Hamas is reaping the fruits, when it paid such a steep price? What did Hamas gain? 50 martyrs..."
Interviewer: "This figure is..."
Salah Al-Bardawil: "I am giving you an official figure. 50 of the martyrs in the recent battle were from Hamas. Before that, at least 50% of the martyrs were from Hamas. So what did Hamas gain from this?"
From looking at the transcript of a longer fragment than what most sources cite, it's clear that Al-Bardawil is talking just about the deaths of May 14th.
Firstly he speaks specifically about 50 Hamas members and another 12 "regular people" which sums up to 62, the number of dead on May 14th.
Secondly, he speaks about the previous days separately and separates them from the 50 dead on May 14th, saying that at least 50% of the dead on previous days were Hamas members.