What is the evidence that the original shadow present at the sun's zenith is not included when calculating the time for Asr (when the shadow of everything becomes equal to its length)?

The time for Dhuhr prayer begins when the sun passes its zenith, a point on which the Muslim community has reached a consensus. The sign of this passing is the increase in shadow after it has reached its minimum length. When you place a stick in the sun and mark the tip of its shadow, a decreasing shadow means the sun has not yet passed its zenith. When the shadow begins to lengthen, it signifies that the sun has passed its zenith and the time for Dhuhr has begun. The length of the shadow at the zenith varies with seasons and locations. In some places and at certain times, the shadow may disappear completely at midday, but the Dhuhr time begins when the shadow starts to reappear towards the east (the direction of sunrise).