Hadith comprehension

What is the grammatical function of “قِطْعَةٌ” in “السَّفَرُ قِطْعَةٌ مِنَ الْعَذَابِ”?

Travel, repayment, service, and sufficiency (Hadith 37–43)

• A preposition
• An imperative verb
• The predicate of the subject
• A conjunction only