A verse in Behukotai says:
The Pesikta Zutrata says:
meaning that the word "Af" was not said regarding Ya'akov Avinu since his bed was perfect. No Yishma'el or Esav descended from him, which would be a cause of "Af" (anger) before Hashem.
The midrash is saying that even though the word "Af" here is being used to mean "even", the second meaning of the word "Af" - anger - is considered to be so bad that Yaakov Avinu's name should not be associated with it due to the perfection of his seed.
We find a similar idea in the Sephardic Birkat Hamazon. In the "רצה והחליצנו" prayer that we say on Shabbat, early Nus'ha'ot would have:
Later editions did not want to have the word אף (or any final-phey for that matter) in the Birkat Hamazon, so the Nusah was changed to:
The Zohar says that "Af" is associated with one of 4 Kelipot we try to keep away from our prayers. See p. 165 here.