Iran

[tartare]

Eyewitnesses who were with him at the time claiming it was a `small missile' that struck the room he was in.
Damage to some windows doors and walls.
Elbit systems has developed a micro-drone that can be launched from a quadcopter - technology certainly exists.
From The Telegraph:

Deep, perhaps unprecedented, Mossad infiltration

Whether the munition was delivered by drone or a different kind of projectile, a launch from inside Iran would suggest deep, perhaps unprecedented, infiltration by Mossad.

“It was a missile,” Khalil al-Hayya, a senior Hamas official, told reporters in Tehran.

“The missile targeted the room where Ismail Haniyeh was present. The impact of the missile caused the windows to shatter and the walls to collapse. Investigation by our Iranian brothers is ongoing.”