Opposite words for affirmation:
negation, disclaimer, disproof, disavowal, question, refutation, dispute, reversal, confutation, rebuttal, contradiction, challenge, denial.
Usage samples:
  1. I was sure that I knew my man; but this strange affirmation in regard to him threw me into terrible doubts.
    "The Good Time Coming", T. S. Arthur.
  2. To this end the whole body of affirmation, persuasion, assumption, suggestion and technique of Christian Science is directed.
    "Modern Religious Cults and Movements", Gaius Glenn Atkins.
  3. The Treaty of Biac- na- Bato has furnished to many the opportunity of renewing the crafty insinuation, nay, bold affirmation, already made by the rebel chiefs, that the religious institutes were the sole cause of the insurrection.
    "The Friars in the Philippines", Ambrose Coleman.
  4. She understood that his nod meant acquiescence with her request, rather than affirmation as to his objecting to her company.
    "Mrs. Red Pepper", Grace S. Richmond.