Ihsan, the hadith of Jibril
أَنْ تَعْبُدَ اللَّهَ كَأَنَّكَ تَرَاهُ، فَإِنْ لَمْ تَكُنْ تَرَاهُ فَإِنَّهُ يَرَاكَ
An ta'buda-llaha ka-annaka tarah, fa-in lam takun tarahu fa-innahu yarak.
"[Ihsan is] to worship Allah as though you see Him, and if you do not see Him, know that He sees you."
Narrated by Umar ibn al-Khattab. Sahih Muslim 8 · Nawawi's Forty · #2
A short reflection
The whole deen distilled into one sentence: worship as if you see Him, because He sees you.