The Quran

وَمِنَ النّاسِ وَالدَّوابِّ وَالأَنعامِ مُختَلِفٌ أَلوانُهُ كَذٰلِكَ ۗ إِنَّما يَخشَى اللَّهَ مِن عِبادِهِ العُلَماءُ ۗ إِنَّ اللَّهَ عَزيزٌ غَفورٌ

Muhammad Asad

and [as] there are in men, and in crawling beasts, and in cattle, too, many hues?¹ Of all His servants, only such as are endowed with [innate] knowledge² stand [truly] in awe of God: [for they alone comprehend that,] verily, God is almighty, much-forgiving.

Cf. 16:13, where the splendour of nature ("the beauty of many hues") is spoken of as an evidence of God's creative power.
I.e., spiritual knowledge, born of the realization that the phenomena which can be observed do not comprise the whole of reality, inasmuch as there is "a realm beyond the reach of a created being's perception" (cf. surah 2, note 3).