The gravity of missing a fast without a valid reason

We must not neglect a fast in Ramadhan without a valid reason because according to the Shari’ah, a Muslim who is sane and mature, MUST fast. Without a valid reason, it is not permissible to leave out a fast. It is a big sin to leave out a fast without any valid reason.

Rasullullah (Sallallaahu Alaihi Wasallam) said: 

"Whoever leaves out a fast of Ramadhan without a reason (valid according to the Shari’ah) and without a (severe / grave) sickness, then fasting for a lifetime will not be sufficient to replace this fast." (Bukhari, Tirmizi, Abu Dawud, Nasai and Ibn Majah)

[English version of "Gravité manque ene roza sans raison valable" – Al Jamiat – Vol. 4 No.14]