Horkheimer discusses this most clearly in “Traditional and Critical Theory” and related passages on critical theory in his early essays. In those texts, the point of critique is not occasional fault-finding, but a sustained, immanent criticism of society that keeps theory tied to suffering and social contradiction.

Best place to look

The single most relevant place is:

  • “Traditional and Critical Theory” (1937) , where Horkheimer defines critical theory as a mode of thought that does not just describe society but challenges it from within.
  • In the same intellectual frame, he says critique must stay connected to the experience of misery and social suffering, so it becomes a continuing practice rather than a one-time judgment.

Why this fits your phrasing

If by “the necessity of criticizing constantly” you mean the idea that philosophy or theory should keep questioning existing society instead of accepting it, that is exactly Horkheimer’s view in these writings. He treats criticism as a way of preventing people from losing themselves in what seems natural or inevitable, rather than as simple complaining.

Useful secondary clue

A good secondary source summary is the Stanford Encyclopedia entry, which explains that Horkheimer’s critical theory works through immanent critique and aims to transform the presentation of social contradictions into a force for change. That is probably the cleanest way to identify the theme you’re asking about.

Where to read first

  1. “Traditional and Critical Theory” for the main argument.
  1. Horkheimer’s 1931 inaugural lecture for an early version of the idea that philosophy must resist mere conformity.
  1. The Stanford Encyclopedia of Philosophy entry on Horkheimer for a concise guide to the theme.

If you want the exact passage, the 1931 lecture and “Traditional and Critical Theory” are the two best starting points.