Hanbal ibn Ishaaq said: I heard Abu ‘Abdullah say: The narration of Ibn Ishaaq cannot be used as proof.
‘Abdullah ibn Ahmad said: He – i.e., Ahmad ibn Hanbal – did not use his reports as evidence when discussing what is Sunnah.
Ayyoob ibn Ishaaq said: I asked Ahmad ibn Hanbal: O Abu ‘Abdullah, if Ibn Ishaaq is the only narrator of the hadith, will you accept it? He said: No, by Allah, for I have seen him putting together the words of many narrators in a single hadith, and not separating the narration of one person from that of another.
Yahya ibn Ma‘een classed him as da‘eef in one report narrated from him. An-Nasaa’i said: He is not qawiy (strong). Ad-Daaraqutni said: The leading scholars differed concerning him, and he is not an authority; rather his narration may be taken into consideration (alongside others)
See: Tahdheeb at-Tahdheeb (9/45).