الدُّعَاءُ مُخُّ الْعِبَادَةِ.
Supplication is the core of worship—one of its most important expressions.
Source: al-Tirmidhi (as cited in the book).
ArabicMeaning
الدُّعَاءُsupplication; calling upon Allah
مُخُّcore, essential part
الْعِبَادَةِworship and devotion
In supplication a person presents a need to Allah, relies upon His power and mercy, and acknowledges dependence upon Him. مُخُّ is figurative here: it points to the importance of supplication in worship rather than a physical part.
A small grammar point: الدُّعَاءُ is the subject (mubtadaʾ), and مُخُّ is the central word of the predicate. مُخُّ الْعِبَادَةِ is an iḍāfah (construct phrase) meaning 'the core of worship,' with الْعِبَادَةِ as its second noun.