And ṭahūr—with the first vowel a—means that by which one purifies oneself. This indicates that tayammum is purifying in itself, not merely something that permits prayer.
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The Prophet, peace and blessings be upon him, said: “Pure earth is a means of purification for the Muslim, even if he does not find water for ten years.” (1)
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It is a substitute for purification with water, and a substitute takes the same ruling as that which it replaces.
And [tayammum is invalidated] by the nullifiers of wudu, and by finding water, even during the prayer, …
His statement, “by the nullifiers of wudu”: this is the second of the nullifiers of tayammum, namely, the nullifiers of wudu—that is, whatever invalidates wudu.
For example, if someone performs tayammum for minor ritual impurity and then urinates or defecates, his tayammum is invalidated, because a substitute takes the same ruling as that which it replaces.
Likewise, tayammum performed in place of major purification is invalidated by whatever necessitates ghusl. This is quite evident.
His statement, “and by finding water”: this is the third of the nullifiers of tayammum: finding water, when one performed tayammum due to the absence of water.
Thus, if one performs tayammum because water is unavailable, it is invalidated upon finding water. But if one performs tayammum due to illness, finding water does not invalidate it, because he may perform tayammum despite the presence of water. Rather, it is invalidated upon recovery, since the factor permitting it—namely, illness—has ceased. Thus, had the author said, “and by the cessation of what permits it,” that would have been more appropriate.
His statement, “even during the prayer”: the word “even” indicates a difference of opinion. When scholars explicitly mention something that is already included in the preceding general statement, this indicates that there is a disagreement concerning it which they needed to note. For his statement, “even during the prayer”