It is reported from Mujahid (may Allah have mercy on him) regarding "Those who humble themselves in their prayers" [Surah al-Mu'minun: 2] that khushu' means stillness in prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3524
Hadith Gradingمحدثین:صحيح
Hadith Takhrij«صحيح۔ اخرجه الطبري في تفسيره 9/ 196»