Brahma Sutras
ॐ यत्रैकाग्रता तत्राविशेषात् ॐ ॥ ४-१-११॥
Meditation is to be undertaken wherever the mind gets concentrated, because there is no specification.
english translation
OM yatraikAgratA tatrAvizeSAt OM ॥ 4-1-11॥
hk transliteration by Sanscript