أُوصِيكَ بِتَقْوَى اللَّهِ، فَإِنَّهُ أَزْيَنُ لِأَمْرِكَ كُلِّهِ
“I advise you to practice taqwa of Allah; it is the finest adornment for all your affairs.”
Taqwa concerns both inner life and outward conduct: sincerity in prayer, honesty in trade, truthfulness in speech, justice at home and restraint in disagreement. Someone cannot treat the rights of others as irrelevant while claiming that piety concerns worship alone.
The فَ in فَإِنَّهُ connects the explanation to the counsel; إِنَّ adds emphasis. كُلِّهِ extends the meaning across all one's affairs rather than one isolated part of life.