The Huffington Post article is based on this 'exclusive' Reuters report dated "BAGHDAD Sun Aug 10, 2014 8:00am EDT", which contains the sentence you quoted, i.e.:
"We have striking evidence obtained from Yazidis...".
However this slightly later article by CNN dated August 11, 2014 (Updated 1746 GMT), quotes a close (but not identical) source, i.e. this time a "spokesman" from the Iraqi human rights ministry rather than the minister himself, as saying,
On Sunday, Iraqi Ministry of Human Rights spokesman Kamil Amin said it was possible that as many as 500 Yazidis had been killed. The ministry had also heard reports -- but had not confirmed -- that some had been buried alive.
"It's difficult to be accurate about these numbers, but initially we have reported 500 Iraqi Yazidis have died from either ISIS direct killings or from starvation and dehydration," Amin told CNN. "We have heard some reports from activists and local journalists that some families were buried alive."
CNN is unable to authenticate reports regarding the Yazidi death toll or the allegation that some were buried alive.
Instead of the original "we have striking evidence" he is now being quoted as "we heard unconfirmed reports".
You asked, "Is this true? If so, where is the evidence?"
This answer cannot tell you whether the reports are true: I posted this answer because it clarifies the evidence on which the claim was made (i.e. that the evidence was, allegedly, unconfirmed reports from activists and local journalists; and that CNN was unable to authenticate the reports).