Jazak Allah khayr, but we also need to remember that not all of the muhadithoon said this about him (rahimahullah). Some graded him as thiqa (trustworthy / reliable). Here is something from Shaykh Jalal Abdualrub:
The Status of Muhammad Ibn Is`haq in Islam
In his scholarly encyclopedia on biographies of notable Muslims entitled, Siyaru A`lami an-Nubalaa, Imam Shams ad-Din adh-Dhahabi (673-748 AH/1274-1347 CE), a major scholar of Hadeeth and Islamic History, stated the following facts about Muhammad Ibn Is`haq, author of, as-Seerah an-Nabawiyyah.
1 – He was born in the year 89 AH (707 CE) and met Anas Ibn Malik, the Prophet’s companion, and az-Zuhri, a major scholar among the second generation of Islam.
2 – Adh-Dhahabi then mentioned these major Hadeeth scholars who stated that Ibn Is`haq was reliable in Hadeeth narrations, grading his narrations as Hasan [‘Hasan’, is the lesser grade of authentic Hadeeths; ‘Sahih’, is the upper grade.]: Yahya Ibn Ma`een and A`hmad Ibn Hanbal [according to the conditions that will soon be mentioned, Allah willing]. Abu Zur`ah stated that Ibn Is`haq was Saduq (truthful), `Ali Ibn Abdullah said that Ibn Is`haq’s narrations are accepted; Ibn `Adi said that Ibn Is`haq is acceptable; and Ibn Idris said that Ibn Is`haq was a Thiqah (reliable, or trustworthy). Also, Imam Abu Zur`ah stated that a group of scholars learned knowledge with Ibn Is`haq, such as Sufyan, Shu`bah, Ibn Uyainah, Ibn al-Mubarak, and so forth. Az-Zuhri, Asim Ibn Umar Ibn Qatadah and adh-Dhahabi also praised Ibn Is`haq’s knowledge in the Maghazi (narration of battles).
3 – Adh-Dhahabi also listed some of the major scholars of Islam who refuted Ibn Is`haq’s reliability in Hadeeth narrations. Imam Malik, for instance, called Ibn Is`haq a liar and Yahya Ibn Sa`eed al-Ansari, as well as, al-A`mash refuted one of Ibn Is`haq’s narrations by saying that he lied. As a general statement, Yahya Ibn Sa`eed graded Ibn Ishâq as being weak in Hadeeth narration. Imam Ahmad Ibn Hanbal discounted the reliability of Ibn Ishâq if he alone narrates a Hadeeth. Also, Imams Yahya Ibn Ma`een (in another narration from him), an-Nasaii and ad-Daraqutni stated that Ibn Ishâq was weak in Hadeeth. The great Imam of Sunnah, Imam A`hmad Ibn Hanbal, also added that Ibn Is`haq’s narrations are not accepted if they are about the Sunan (Pl. for Sunnah; [yet, Craig Ibn Winn claims that Ibn Is`haq’s Seerah is a Sunnah book!]), stating that even [in the rare occasions] where Ibn Is`haq clearly stated that he heard a Hadeeth from his teacher, he would often contradict other narrators. Therefore, and as Imam A`hmad stated, if Ibn Is`haq alone reports a Hadeeth, then that narration is not accepted. Adh-Dhahabi also stated that if a narration that Ibn Is`haq reports contradicts other [more established] narrators, then Ibn Is`haq’s narration is rejected.
4 – Adh-Dhahabi listed some of the reasons why Ibn Is`haq was considered weak regarding Hadeeth narration, as follows.
A – Imam A`hmad Ibn Hanbal stated that Ibn Is`haq was a Mudallis , and in another occasion, he said that Ibn Ishaq’s Tadlees (v. for Mudallis) was substantial. Imam A`hmad also said that Ibn Is`haq did not care from whom he collected Hadeeth.
[Ibn Is`haq often started his narrations by saying, “Those whom I trust narrated to me”, or “Some men from this city told me”, etc. He also would collect Hadeeths from unreliable narrators and hide the name of his teacher by saying, “So and So said”, meaning the teacher of his teacher, who may be trustworthy, so that the Hadeeth narration is not rejected if the name of his own teacher is specified. However, whenever Ibn Is`haq said, “So and so said to me”, he would not lie.]
B – Imam Ibn Numair said that Ibn Is`haq reported false Hadeeths from unknown narrators.
C – Adh-Dhahabi concluded by saying that among the worst errors made by Ibn Is`haq is that he used to record narrations he collected from anyone, and thus, did not have Wara` [‘Wara’’, pertains to meanings of having fear from Allah in a profound way that guides one to stay away from suspicious things.] in this regard, may Allah forgive him.
5 – How Ibn Is`haq’s narration should be treated is summarized in this statement from Imam Ibn Numair, “If he narrates a Hadeeth from teachers he directly heard from and who are known to be truthful, then his Hadeeth is from the grade Hasan because he is truthful.” Yet, Imam A`hmad stated that if Ibn Is`haq is the only narrator of that Hadeeth, then his narration is discounted. And the key words to look for here, for Ibn Ishaq’s narration not to be dismissed outright, are, “If Ibn Is`haq says, ‘So and so narrated to me’, then he did hear that narration.’ Otherwise, if he says, ‘So and so said’, then the narration is rejected.’” Meaning, Ibn Is`haq would not lie; if he states that he heard the Hadeeth from his teacher, then his assertion is accepted.
Definition: A Hadeeth is a narration that starts with the collector of Hadeeth narrations, such as al-Bukhari or Muslim, wherein the collector names the teacher from whom he heard the Hadeeth, who also names his teacher, and so forth, until the chain reaches the Prophet’s companion and then the Prophet, peace be upon him.
A Summary of How Muslim Scholars Treated Ibn Is`haq’s Hadeeth Narration
For a Hadeeth reported by Ibn Is`haq to be accepted as a Hasan Hadeeth, which is the lesser grade of authentic Hadeeths, Ibn Is`haq must declare that he heard the narration directly from his teacher, provide a reliable chain of narrators throughout the chain of narration until it reaches the companion or the Prophet, and then his narration cannot contradict a narration reported by a more established narrator or group of narrators. Imam A`hmad added that Ibn Is`haq should not be the only narrator for a Hadeeth, otherwise, his narration is rejected.