950/1 - Anas (may Allah be pleased with him) reported: Some Companions happened to pass by a funeral procession and they spoke in praise of the deceased. The Prophet (may Allah’s peace and blessings be upon him) said: “Guaranteed!” Then, they passed by another funeral procession and they spoke ill of the deceased. The Prophet said: “Guaranteed!” ‘Umar ibn al-Khattāb said: “What is guaranteed?” He replied: “You spoke in praise of this one, so Paradise has been guaranteed for him, and you spoke ill of this one, so Hellfire has been guaranteed for him. You are Allah’s witnesses on earth.” [Narrated by Al-Bukhāri and Muslim]
951/2 - Abu al-Aswad reported: I came to Madīnah and sat with ‘Umar ibn al-Khattāb (may Allah be pleased with him). A funeral passed by and the deceased was praised. ‘Umar said: ‘Guaranteed.’ Then, another funeral passed by and the deceased was praised; whereupon ‘Umar again said: ‘Guaranteed.’ Then, a third funeral passed by and the deceased was dispraised; whereupon ‘Umar said: ‘Guaranteed.’ I asked: “O Commander of the Believers, what is guaranteed?” He said: “I said what the Prophet (may Allah’s peace and blessings be upon him) said: ‘Allah admits to Paradise any Muslim whose goodness is testified by four people.’ We said: ‘What about three people?’ He replied in the affirmative. We said: ‘What about two?’ He replied in the affirmative. Then, we did not ask him about one person.’” [Narrated by Al-Bukhāri]
1) The believers are Allah’s witnesses on earth. So, if they praise a person, Paradise becomes certain for him; and if they speak ill of him, Hellfire becomes certain for him.
2) According to the belief of Ahl-us-Sunnah, we should not confirm that anyone will enter Paradise or Hellfire, unless this is established by certain proofs, like in the case of the ten Companions given glad tidings that they would enter Paradise, as well as Abu Lahab who was given tidings that he would enter Hellfire. But when Muslims speak well of someone, this is a good sign and it is hoped he would enter Paradise, without decisively saying that he would.