The preacher, Mawlawi Rayhan, was a supporter of the Western-backed Afghan government which Taliban militants are trying to bring down.
No militant group has so far claimed responsibility for the attack.
The explosion hit the Al-Taqwa mosque at a time when dozens of men had gathered for prayers. At least 16 men, all civilians, were injured.
The death toll could rise, said a second official.
“We have rushed ambulances to the blast site,” said a senior official in Kabul.