Bukhari

Hadith Commentary:
If Ibn Sayyad was not the Dajjal, then the Prophet (sallallahu alayhi wa sallam) would certainly have prevented Umar (radi Allahu anhu) from swearing an oath about him.

Here, an objection arises: it has already been mentioned above in the Book of Funerals that when Umar (radi Allahu anhu) wanted to kill him, the Prophet (sallallahu alayhi wa sallam) said, "If he is the Dajjal, you will not be able to kill him; and if he is not the Dajjal, then killing him would not be better for you."

From this, it appears that the Prophet (sallallahu alayhi wa sallam) himself was uncertain about him being the Dajjal. Then why did he not object when Umar (radi Allahu anhu) swore an oath about it?

The answer is that perhaps at first the Prophet (sallallahu alayhi wa sallam) was uncertain about him being the Dajjal, but when Umar (radi Allahu anhu) swore this oath, it became clear that he was indeed the Dajjal.

Abu Dawud narrated from Ibn Umar (radi Allahu anhu) that he used to swear and say, "Indeed, Ibn Sayyad is the Masih Dajjal." And it is possible that the Prophet (sallallahu alayhi wa sallam) did not object to Umar (radi Allahu anhu) because Ibn Sayyad was also one of those thirty Dajjals whose emergence is mentioned in another hadith. In this sense, his being a Dajjal was certain.

And Muslim narrated the story of Tamim Dari (radi Allahu anhu), that he saw the Dajjal on an island and related this story to the Prophet (sallallahu alayhi wa sallam). And Muslim narrated from Abu Sa'id (radi Allahu anhu) that he and Ibn Sayyad traveled together up to Mecca. Ibn Sayyad said, "What is wrong with the people that they consider me to be the Dajjal? Did you not hear the Prophet (sallallahu alayhi wa sallam) say that the Dajjal will not enter Mecca and Medina?" I said, "Indeed, I have heard that." He said, "Did you not hear the Prophet (sallallahu alayhi wa sallam) say that he will have no children?" I said, "Indeed, I have heard that." Ibn Sayyad said, "I have children, and I was born in Medina, and now I am going to Mecca."

And Abu Dawud narrated from Jabir (radi Allahu anhu) that Ibn Sayyad disappeared during the incident of Harrah. Some said he died in Medina and people prayed over him.

In one narration, Ibn Sayyad said, "Indeed, I do know the Dajjal, and I know where he was born, and I also know where he is now." Upon hearing this, Abu Sa'id al-Khudri (radi Allahu anhu) said, "Woe to you, may you perish all day! You have again cast doubt."

In another narration, with a sound chain from Abd al-Razzaq, from Ibn Umar (radi Allahu anhu), it is stated that one of Ibn Sayyad's eyes had become swollen. I asked him, "Since when has your eye been swollen?" He said, "I do not know." I said, "You are lying; your eye is in your head and you say you do not know." Upon hearing this, he said, "Pass your hand over your eye!" and three times made a sound like a donkey.

I mentioned this to Umm al-Mu'minin Hafsa (radi Allahu anha). She said, "You should avoid him, for I have heard people say that when the Dajjal is angered, he will emerge." Thus, the Companions remained in doubt as to whether Ibn Sayyad was the Dajjal or not.

Imam Ahmad narrated from Abu Dharr (radi Allahu anhu): "If I were to swear ten times that Ibn Sayyad is the Dajjal, that would be better for me than swearing once that he is not the Dajjal."

(Ibn Sayyad was also a kind of Dajjal, but the Promised Dajjal is the one who will appear near the Day of Judgment.)

Hadith Commentary:
➊
The Messenger of Allah (sallallahu alayhi wa sallam) was infallible and protected from error, therefore, his remaining silent upon witnessing an action is evidence for its permissibility.
If Ibn Sayyad were not the Dajjal, then the Prophet (sallallahu alayhi wa sallam) would certainly have prevented Umar (radi Allahu anhu) from swearing an oath regarding him.
From the hadith of Tamim al-Dari (radi Allahu anhu), it is understood that Ibn Sayyad is not that Dajjal whom Isa (alayhis salam) will kill. Therefore, the Prophet’s (sallallahu alayhi wa sallam) silence upon Umar’s (radi Allahu anhu) oath establishes the reality that Ibn Sayyad is also one of those Dajjals who will appear before the Day of Resurrection, but regarding the greater Dajjal (Dajjal Akbar), the Prophet (sallallahu alayhi wa sallam) was certain that he is among the signs of the Hour and will only appear near the Day of Resurrection.
And Allah knows best.
In one hadith, it is mentioned that Umar (radi Allahu anhu) intended to behead him, but the Messenger of Allah (sallallahu alayhi wa sallam) said:
“If he is indeed the Dajjal, then you will not be able to overpower him, and if he is not that Dajjal, then there is no benefit in killing him.”
(Sahih al-Bukhari, al-Jana’iz, Hadith: 1354)

➋
From this hadith, it is evident that the Messenger of Allah (sallallahu alayhi wa sallam) had doubt regarding Ibn Sayyad—whether he was the Dajjal or not.
It is possible that initially the Prophet (sallallahu alayhi wa sallam) was certain, but later, due to circumstances and indications, he became doubtful and considered it necessary to investigate further.
In any case, the Messenger of Allah (sallallahu alayhi wa sallam) was certain that he was not the greater Dajjal (Dajjal Akbar), and was also certain that he was one of the thirty Dajjals who will appear before the Day of Resurrection.
And Allah knows best.

Hadith Commentary: Benefits and Issues: There is no doubt whatsoever regarding Ibn Sayyad being a Dajjal. The only difference of opinion is whether he is the very Dajjal al-Akbar, that is, the Messiah Dajjal, or a lesser Dajjal (Dajjal Asghar). The Dajjal al-Akbar will appear in the last era. Some hold the view that he (Ibn Sayyad) will appear in the form of Dajjal al-Akbar, while others believe that the final Dajjal is someone else, as is mentioned in the hadith of Tamim al-Dari. However, even from that hadith, it is evident that Dajjal al-Akbar has already been born and exists, but his appearance will be in the last era. At that time, ‘Isa (alayhis salam) will kill him.