Can a person be regarded as a fasiq (sinner) if he has only missed a single prayer?
Yes, even a single prayer that has been made qadah without a valid reason, is regarded as fisq (corruption) and a major Sin, and the offender is deserving of Jahannam.
Here we should take a moment to reflect and think about how many prayers we have missed since we arrived at the age of adulthood, and whether we will be forgiven for missing all the prayers we have missed. We learn from the above mentioned ruling that a single missed prayer has severe consequences.
We should realise the importance of performing every prayer and should ask ourselves whether we have a justified reason for missing any of the prayers that we have missed, after all the Prophet Muhammad ﷺ and many of the companions did not let a Salah become qadah even during Jihad.
And Allah ﷻ knows the best
Imam Ahmad Rida Khan رحمة الله تعالى عليه
Translation by Dr Musharraf Hussain Al-Azhari Translator of Majestic Quran, www.majesticquran.co.uk