I remember that I once read a sutta by the buddha where he mentioned to do things slowly while being mindful in order to recognize each step. Any ideas which sutta it could be?
There is no sutta or commentary which taught to meditate slowly. The action speed depends on the practitioner skill level, some uneducated practitioner (may be) need to action slow.
But there are many sutta taught to meditate in order. And There are some commentary which have the example of the practitioner who always repeatedly his action when he forget to meditate mindfulness.
However, when we begin to do some very hard new thing carefully, mindfully. we will be automatic slow down ourselves. Because if we do it over fast we will cross the step, mistake the practice's sequences.