The Kitáb-i-Aqdas - The Most Holy Book
Index term: Obligatory Prayer, under exemptions during; Illness
We have commanded you to pray and fast from the beginning of maturity; this is ordained by God, your Lord and the Lord of your forefathers. He hath exempted from this those who are weak from illness or age, as a bounty from His Presence, and He is the Forgiving, the Generous. God hath granted you leave to prostrate yourselves on any surface that is clean, for We have removed in this regard the limitation that had been laid down in the Book; God, indeed, hath knowledge of that whereof ye know naught. Let him that findeth no water for ablution repeat five times the words "In the Name of God, the Most Pure, the Most Pure", and then proceed to his devotions. Such is the command of the Lord of all worlds. In regions where the days and nights grow long, let times of prayer be gauged by clocks and other instruments that mark the passage of the hours. He, verily, is the Expounder, the Wise.
He hath exempted from this those who are weak from illness or age
The exemption of those who are weak due to illness or advanced age from offering the Obligatory Prayers and from fasting is explained in Questions and Answers. Bahá'u'lláh indicates that in "time of ill-health it is not permissible to observe these obligations" (Q&A 93). He defines old age, in this context, as being from seventy (Q&A 74). In answer to a question, Shoghi Effendi has clarified that people who attain the age of seventy are exempt, whether or not they are weak.
Exemption from fasting is also granted to the other specific categories of people listed in the Synopsis and Codification, section IV.B.5. See notes 20, 30 and 31 for additional discussion.
Question: Concerning fasting and obligatory prayer by the sick.
Answer: In truth, I say that obligatory prayer and fasting occupy an exalted station in the sight of God. It is, however, in a state of health that their virtue can be realized. In time of ill-health it is not permissible to observe these obligations; such hath been the bidding of the Lord, exalted be His glory, at all times. Blessed be such men and women as pay heed, and observe His precepts. All praise be unto God, He who hath sent down the verses and is the Revealer of undoubted proofs!