Moving an obstructive pebble once is excused; repeatedly cracking the fingers without need is disliked; verbally returning a greeting is speech that invalidates prayer.
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Applying the rulings to examples
Disliked Acts in Prayer: Movement, Dress, Turning and Validity
Moving an obstructive pebble once is excused; repeatedly cracking the fingers without need is disliked; verbally returning a greeting is speech that invalidates prayer.