According to Haq's son, Hamidul Haq told media that his father was attacked while he was resting alone in his room as his driver Haqqani had gone out for some work.
Police says that the motive behind the killing was not immediately known and no group has so far claimed responsibly for the attack.
"They were all heroes, of course", Haq said then of his students who took part in militant resistance.
Earlier reports had emerged suggesting that Maulana Samiul Haq was shot dead by gunmen riding a motorcycle. "He was no longer alive", Hamid said while talking to Geo News. "We lost a great scholar and religious leader today", he said from China, where he was on an official visit.
Jamiat Ulema-e-Islam (JUI-S) chief Maulana Samiul Haq has been stabbed to death in Rawalpindi on Friday evening, his son confirmed the report.
He headed a political party and was twice elected as member of the Senate, the upper house of Pakistan's parliament.
Prime Minister Imran Khan strongly condemned the martyrdom of Maulana Samiul Haq.
His Dar-ul-Uloom Haqqania seminary educated Taliban stalwarts including Mullah Omar and Jalaluddin Haqqani.
In a statement from Beijing, where he is now on visit to China, the Prime Minister directed to investigate the incident and bring the perpetrators to justice.
A spokesman for the military condemned the "assassination" and expressed "grief and condolences" to his family.
Minister for Human Rights Shireen Mazari said the brutal murder of Maulana Samiul Haq was "utterly condemnable". Former prime minister Nawaz Sharif condemned the martyrdom of Maulana Sami and expressed vast grief over it. Khan had been criticized for supporting Haq, including donating funds to the Haqqania seminary when he was head of a provincial government.