Hazrat Moulana Ashraf Ali Thanwi (rahmatullahi ‘alaih) once mentioned the following:
At the time when engaging in the zikr of Allah Ta‘ala, one should not allow the mind to drift from the zikr one is making, as this will cause the focus and presence of mind to be lost. Rather, one should merely concentrate and focus on the name of Allah Ta‘ala and trying to keep His presence at heart. Making zikr in this manner will prove extremely beneficial.
(Malfoozaat Hakeemul Ummat 23/318)