On his return, he saw that Maulana Sami was lying in his bed in a pool of bed. The JUI-S chief was injured and was being taken to the hospital when he succumbed to his wounds on the way.
He said the government funded Haq's seminary for institutionalising modern curriculum, while the slain cleric had also backed the government for the anti-polio drive.
Prime Minister Imran Khan, who is now in China for an official visit, said the country had been deprived of an important religious leader whose services would always be remembered.
A spokesman for the military condemned the "assassination" and expressed "grief and condolences" to his family.
Speaking to ARY News, the deceased's son Hamid ul Haq said his father was stabbed to death at his Rawalpindi residence.
In a statement from Beijing, where he is now on the visit to China, the prime minister directed to investigate the incident and bring the perpetrators to justice.
Maulana Samiul Haq was a religious scholar and former Senator.
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Punjab Chief Minister Usman Buzdar has also denounced the assassination and sought a report from the Inspector General Punjab Police.
DW interviewed Haq in 2015, during a visit to Haqqania. He said that Haq's services would always be remembered.
Former interior minister Ahsan Iqbal said the bad news of the assassination was very tragic.
He had also served as a member of the Senate.
Haq's killing has come when religio-political parties in Pakistan are protesting on the streets against the Supreme Court's decision to acquit Asia Bibi.
He recalled that Maulana Samiul Haq was one of the few clerics who had openly supported the Khyber Pakhtunkhwa (KP) government in running polio eradication campaign.