RT.com. It was the Russians. However, this is reported by Fars. It’s probably untrue.
Additional evidence is cited in the article:
“[J]jihadists claimed on social media that Al-Nasr had been killed.
Other photos published by the terrorist organization show the alleged graves of Al-Nasr and two other militants who were killed in an airstrike.
The death of Sanafi Al-Nasr, whose real name was Abdul Mohsen Abdallah Ibrahim Al-Charekh, was also confirmed by the Britain-based Syrian Observatory for Human Rights, who claimed the jihadist leader was killed on Thursday.”