Second Volume, 63Rd Letter

If a person misses his daily prayers of salat [1] because of illness and does not know the number of prayers he has missed, he makes qada' [2] of the prayers he has missed in lieu of the supererogatory prayers of salat such as (those prayers of salat termed) Tahajjud and Ishraq as well as the sunnats [3] of the five daily prayers of salat; the prayers of salat he performs with the intention of qada' though he has finished his debts of salat will become supererogatory. He will be given the thawab (blessings promised) for supererogatory worships. For it is not necessary to make a certain niyyat (intention) for supererogatory prayers of salat performed at certain times. Prayers of salat made qada' at certain times become supererogatory prayers belonging to those times.

[It is a grave sin to omit the five daily prayers of salat, that is, not to perform them (within the times allotted to them) without any 'udhr (a good excuse for doing or not doing something). Fawt [missing or omitting them] because of illness or another 'udhr is not sinful. For this, it is necessary to make qada of them, (that is, to pay one's debts pertaining to salat), instead of the sunnats of the five daily prayers of salat except the sunnat of morning salat. That these sunnats are supererogatory prayers is explained in the books Jawhara and Futuh-ul-ghayb as well as in Nawadir-i-fiqhiyya, by Muhammad Sadiq (rahmatullahi alaih).

[1] salat: i) prayer; (with salam)= salawat; ii) ritual prayer of at least two rak'as; "namaz", in Persian; salat janaza: funeral prayer.
[2] qada': i) decree of a qadi (Muslim judge); ii) performance of an 'ibada after its due time.
[3] sunnat: i) (when used alone) The Shari'at; ii) (when used together with the name Book) The hadith of the Prophet. iii) (when used together with the word Fard) Any action, word or thought liked and commanded by the Prophet.