Surah 13. Ar-Ra'd, Ayah 21

This refers to all ties arising from human relationships - e.g., the bonds of family, responsibility for orphans and the poor, the mutual rights and duties of neighbours - as well as the spiritual and practical bonds which ought to exist between all who belong to the brotherhood of Islam (cf. 8:75 and the corresponding notes). In its widest sense, the phrase "what God has bidden to be joined" applies to the spiritual obligation, on the part of man, to remain conscious of the unity of purpose underlying all of God's creation, and hence - according to Razi - man's moral duty to treat all living beings with love and compassion.

That is, join faith with practice, love of God with love of man, and respect for all Prophets alike, i.e., follow the right Religion, and not odd bits of it.