It's good to know a little about Ibn Ishaq as you may come across quotes from his work on various websites across the internet. The fact remains his biographical work is not binding on Muslims and each account in his work needs to be looked at in order to ascertain whether it's reliable or not. Just because it's in his biographical work does not mean it is reliable. Do not be fooled by Islamophobic spin.
Ibn Ishaq (d. 150/767)....was a very controversial figure. Malik, Ibn al Qattan, Ibn Hanbal, and others considered him highly unreliable because he accepted hadiths from questionable narrators as well as Christians and Jews. But Ibn Shu'ba felt he was impeccably reliable... [Hadith, Muhammad's Legacy in the Medieval and Modern World, Jonathan A C Brown, Oneworld Publications, Kindle, p86]
And here's further insight into who Ibn Ishaq was, his and his methodology:
Let us agree first that Ibn Ishaq is a controversial figure in the Muslim history. In what follows I will focus on his life and his work on the biography of the Prophet (peace be upon him).
His name is Muhammad ibn Ishaq ibn Yasar. He was born in Madinah about 80 AH/699 AD and died in
Ibn Ishaq was a contemporary of the second generation of traditionists such as al-Zuhri, Asim ibn Umar ibn Qatadah and Abd Allah ibn Abi Bakr. He devoted himself to the study of Hadith from his youth. At the age of thirty he traveled to
Yahya ibn Ma`in, an early Hadith authority, said: "Ibn Ishaq is firm in tradition." Abu Zur`ah (d. 281AH/860AD) said: "Older scholars drew from him and professional traditionists tested him and found him truthful."
Ibn al-Madini said: "Prophetic tradition originally lay with 6 men; then it became the property of 12, of whom Ibn Ishaq is one."
Al-Bukhari quoted him as an authority and Muslim cited him often.
On the other hand, other scholars accused him of being a Qadari and a Shi`i. In response to this accusation, scholars said that the narrations which might be interpreted as a support for Shi`isim were reported by Ibn Ishaq in the form of stories not as a support.
Ibn Ishaq excelled in the field of Hadith, Fiqh, Tafsir and Maghazi (the Prophet's wars).
Ibn Ishaq's biography of the Prophet is titled: The Book of the Campaigns or The Book of the Campaigns and the Prophet's Biography or The Book of the Beginning of the Campaigns. Al-Baka', a pupil of Ibn Ishaq, made two copies of the whole book, one of which must have reached Ibn Hisham (d. 218AH/797AD) whose text, abbreviated and annotated is the main source of our knowledge of the original work.
Throughout his work, Ibn Ishaq precedes every statement with the word za`ama or za`amu, he (they) alleged). It carries with it more than a hint that the statement may not be true, though it might be sound. This attitude reflects Ibn Ishaq's caution and fairness.
The phrase 'God knows best' speaks for itself and needs no comment. It is sometimes when the author records two conflicting traditions and is unable to say which one is correct. Another indication of the author's scrupulousness is the phrase 'God preserve me from attributing to the prophet's words which he did not use."
It is not always Ibn Ishaq's Sirah which is attacked but the man himself. Ibn Ishaq had another book titled Al-Sunan which if it ran counter to the schools of law that were in the process of development, the author would hope to escape strong condemnation.
By now, we can conclude that Ibn Ishaq is the main source of our knowledge about the Prophet as all later works relied on the narrations contained in his book which survives only in the recension of Ibn Hisham.
I hope this answers your query.
Note: This answer is based on Alfred Guillaume's introduction to his Translation of Ibn Ishaq's Sirat Rasul Allah (The Life of Muhammad), published by Oxford University Press, 1955. [OnIslam]
Ehteshaam Gulam writes:
Several critics of Islam say that Ibn Ishaq's "Sirat Rasulallah" - "Life of the Prophet of Allah" is considered the most authentic biography of Prophet Muhammad. This is certainly false. While it is true that Sirat Rasulallah is the oldest and earliest biography of the Prophet-- no Muslim accpets Ibn Ishaq to be 100% true, inspired, a sunnah book or reliable. Most of the material of the Sira as we'll see has been rejected by hadith collectors such as Bukhari, etc. What several critics of Islam and Prophet Muhammad (peace be upon him) fail to realize is that there is a huge difference between Sirah (Biography of the Prophet) and Hadith (the sayings and actions of Prophet Muhammad). The Seerah is a collection of narrations about the people and events surrounding the Prophet (SAW) arranged in a chronological order. The amount of rigor put into authenticating and analyzing the chains and narrators  of an incident or event that is found in the Seerah is far lower than when a narration is used in the Sunnah or Hadith. Only the top of the top narrations, namely Sahih  or Hasan  are used in the books of Hadith and Sunnah. As for Seerah this is not the case, the narrations used include all the authentic and acceptable ones, along with ones with weaknesses. The reason for including these weaker narrations is in order to fill in gapes or holes in the story. Muslims accept the hadith 100%. Muslims however don't accept the Sirah 100%--- rather Muslim scholars question 70% of the material found in the works of Al-Waqidi, Ibn Sa'd, Ibn Ishaq, etc. These were more or less historians-- they were not hadith collectors. Ibn Ishaq, Al-Waqidi, Ibn Sa'd and Al-Tabri all operated outside of f all the sciences of Islam and isnaad (Arabic for chain of transmitters). Ibn Is’haaq’s specialty was seerah therefore he was abandoned by the scholars of hadeeth (such as Bukhari and Muslim) when it came to narrating hadeeth and a reason for this might be because he might include those weaker narrations while he narrated the hadeeth.
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