Hadith.1431 - Muhammad ibn Muslim narrates that Imam Muhammad ibn Ali Al-Baqir (as) said:
"I asked him about a man who became ill and thus did not perform the supererogatory (nafilah) prayers.
Imam (as) said: 'O’ Muhammad, they are not obligatory. If he makes them up, it is a good deed he performs, and if he does not, there is no obligation upon him.'"