1. There are certain surahs which have been prescribed to be recited at specific times during the night and day or during certain days of the week. It is mustahab for one to recite these surahs at their prescribed times.
b) One should recite the three Quls (i.e. Surah Ikhlaas, Surah Falaq and Surah Naas) thrice every morning and evening.
عن عبد الله بن خبيب رضي الله عنه قال: خرجنا في ليلة مطيرة وظلمة شديدة نطلب رسول الله صلى الله عليه و سلم يصلي لنا قال فأدركته فقال قل فلم أقل شيئا ثم قال قل فلم أقل شيئا قال قل قلت ما أقول؟ قال قل هو الله أحد والمعوذتين حين تمسي وتصبح ثلاث مرات تكفيك من كل شيء (ترمذي رقم 3575)
Hazrat Abdullah ibn Khubaib (radhiyallahu ‘anhu) reports, “On one occasion during a dark, rainy night we set out in search for Rasulullah (sallallahu ‘alaihi wasallam) so that he may lead us in salaah. I then met Rasulullah (sallallahu ‘alaihi wasallam) and he told me, “Recite.” But (not knowing what to recite) I did not recite anything. He told me for the second time, “Recite.” But (not knowing what to recite) I did not recite anything. He told me for the third time, “Recite.” I then asked, “What should I recite, O Rasulullah (sallallahu ‘alaihi wasallam)?” Rasulullah (sallallahu ‘alaihi wasallam) replied, “Recite Surah Ikhlaas and Muawwazatain (i.e. Surah Falaq and Surah Naas) in the morning and evening thrice, it will suffice you as a protection from all harm.”