http://www.islam-qa.com/en/ref/26230/door
- Imam Tabari said in the Qur'an, Allah says in Surah Baqarah vers 45:
And seek help in Patience and salah and truly it is extremely difficult except for al-Khashi'oun...
In this ayah, Allah dispraises those who do not have khushoo' and the one who doesn't cannot fulfil the salah properly and so it is wajib.
- Imam Qurtubi said it is fard in salah because Allah says the believers are those with khushoo' as sated above.
- Sheikhul Islam ibn Taymiyyah says khushoo' is wajib in salah (vol. 22 page 553)
- Ibn al Qayyim said in Kitab al Wabil page 24 that the ulema of the salaf agree that to seek khushoo' is fard in salah.
If i know someone is waiting for me at the door and i gotta open it, verily it harms to have concentration in the salah.