Purification is the key to prayer. The Prophet, may Allah’s peace and blessings be upon him, said,
“None safeguards the ablution save a believer.” (Ahmad)
This is because a servant could well invalidate his state of purification with our even being aware of it, therefore safeguarding the state of ablution for prayer is a proof that faith has settled firmly in the heart.
The Legacy of the Prophet Ibn Rajab Al-Hanbali p 26