Understanding and application

Which word is the muḍāf ilayh in مُخُّ الْعِبَادَةِ?

Intention, sincere goodwill, and a first nominal sentence (Hadith 1–7)

• مُخُّ
• الدُّعَاءُ
• الْعِبَادَةِ
• هُوَ