What is Ihsaan (Perfection in Worship)?
Ihsaan is to be conscious that Allah sees us during worship.
Evidence from the Noble Qur'an
{Who sees you (O Muhammad) when you stand up (alone at night for Tahajjud prayer)}, {Soorah ash-Shu'araa, Verse 218]
Evidence from the authentic Sunnah
((Ihsaan is to worship Allah as if you are seeing Him, yet truly He is seeing you)), [Transmitted by Muslim]