Bhagavad Gita Chapter 8.13
oḿ ity ekākṣaraḿ brahma
vyāharan mām anusmaran
yaḥ prayāti tyajan dehaḿ
sa yāti paramāḿ gatim
Translation
After being situated in this yoga practice and vibrating the sacred syllable om, the supreme combination of letters, if one thinks of the Supreme Personality of Godhead and quits his body, he will certainly reach the spiritual planets.