Three years ago, I vowed to fast three days each month my entire life.
I fear that (Part No. 23; Page No. 254) as time goes past, I will grow too weak to fast due to age. Is it permissible for me to slaughter a sacrifice or to feed needy people?
Your vow is to perform an act of obedience which the Shari`ah regards fulfilling it as obligatory. Allah (Exalted be He) states,
They (are those who) fulfil (their) vows, and they fear a Day whose evil will be wide-spreading.
It was authentically reported that the Prophet (peace be upon him) said,
Whoever vows to obey Allah should obey Him; and whoever vows to disobey Allah should not disobey Him.
You should continue fulfilling your vow as long as you can.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.